This article is about the type of website. For the record of web server activity, see server log.
A weblog, or simply a blog, is a website which contains periodic, reverse chronologically ordered posts on a common webpage. Such a web site would typically be accessible to any Internet user. Part of the reason "blog" was coined and commonly accepted into use is the fact that in saying "blog", confusion with server log is avoided. Individual posts (which taken together are the blog or weblog) either share a particular theme, or a single or small group of authors.
The totality of weblogs or blog-related webs is usually called the blogosphere. The format of weblogs varies, from simple bullet lists of hyperlinks, to article summaries with user-provided comments and ratings. Individual weblog entries are almost always date and time-stamped, with the newest post at the top of the page. Because links are so important to weblogs, most blogs have a way of archiving older entries and generating a static address for individual entries; this static link is referred to as a permalink. The latest headlines, with hyperlinks and summaries, are offered in weblogs in the RSSXML-format, to be read with a RSS feedreader.
Before blogging became popular, digital communities took many forms, including Usenet, email lists and bulletin boards. In the 1990's electronic form software, such as WebX, created running conversations with threads. Many of the terms from weblogging were created in these earlier media. For example troll, a person who disrupts a discussion, dates back to Usenet. The word "thread" from email lists and usenet as well. The verb "to post" from electronic bulletin boards.
Blogging combined the personal web page, with tools to make linking to other pages easier, specifically blogrolls and trackbacks, as well as comments. This way, instead of a few people in control of threads on a forum, or anyone able to start threads on a list, there was a moderating effect that was the personality of the weblogs owner.
"Weblogs are often-updated sites that point to articles elsewhere on the web, often with comments, and to on-site articles. A weblog is kind of a continual tour, with a human guide [whom] you get to know. There are many guides to choose from, each develops an audience, and there's also comraderie and politics between the people who run weblogs, they point to each other, in all kinds of structures, graphs, loops, etc." -- Dave Winer, 
The shorter version, "blog," was coined by Peter Merholz who in April or May of 1999 broke the word weblog into the phrase "wee' blog" in the sidebar of his weblog . This was interpreted as a short form of the noun  and also as a verb, to blog, meaning "to edit one's weblog or a post to one's weblog". Usage spread during 1999 and the word was popularized by Pyra's creation of their weblog service Blogger. As of March 2003, the Oxford English Dictionary included the terms weblog, weblogging and weblogger in their dictionary. 
One of the pioneers of the tools that make blogging more than merely websites that scroll is Dave Winer of the Berkman Center at Harvard University. One of his most important contributions was the creation of servers which weblogs would "ping" to show that they had updated. This made it possible for blogs to easy "trackback" other blogs that referenced a particular story, a key feature of blogging.
In early 2002, blogs began to spring up to support the invasion of Iraq, these "war bloggers" were primarily from the right end of the spectrum, and included Instapundit and Little Green Footballs. The first "blog" driven controversy is probably associated with the fall of Trent Lott, where bloggers found quotes from his previous speeches which were taken to be racist, and "kept the story alive" in the press.
By this point blogging is enough of a phenomenon that howto manuals had begun to appear, primarily focusing on using the tools, or creating content. But the importance of a blog as a way of building an electronic community had also been written on, as had the potential for blogs as a means of publicizing other projects.
Through 2003, weblogs gained increasing notice and coverage for their role in breaking, shaping or spinning news stories. The triggering event was the sudden emergence of an opposition to the Iraq war which was not rooted in the traditional anti-war left. The blogs which gathered news on Iraq, both left and right, exploded in popularity, and Forbes magazine covered the phenomenon. The use of blogs by political candidates, particularly Howard Dean and Wesley Clark cemented their role as a news source, while the increasing number of experts who blogged, including Daniel Drezner and J. Bradford DeLong gave the blog world a cachet among regular journalists.
The Iraq War was the first "blog war" in another way - bloggers in Baghdad gained wider readership, and one published a book of his blog. Reading the thoughts of people who were "on the spot" provided a counterpoint, if not a counterweight, to official news sources. Blogs were often used to draw attention to obscure news sources, for example posting links to the traffic cameras in Madrid as a huge anti-terrorism demonstration filled the streets in the wake of the M11 attack. Bloggers would often provide nearly instant commentary on televised events, which became a secondary meaning of the word "blogging", such as "I am blogging Rice's testimony", i.e. "I am writing my reactions to Rice's testimony as I watch it."
In 2004, the role of blogs became increasingly mainstream, as political consultants, news services and candidates began using theme as tools for outreach and opinion formation. Minnesota Public Radio broad cast a program by Christopher Lydon and Matt Stoller called "The Blogging of the President", which covered the transformation in politics that blogging seemed to presage. The Columbia Journalism Review began regular coverage of blogs and blogging. Anthologies of blog pieces began to reach print, and blogging personalities began appearing on radio and television.
Many more advanced bloggers prefer to generate their blogs by using server-side software tools such as Movable Type, WordPress, and Manila to publish on their own Web site or a third party site, or to host a group of blogs for a company or school. Such programs provide greater flexibility and power, but require more knowledge. If they provide a Web interface for editing, server-based systems may reduce the ease of creating and editing text for travellers, some of whom like to produce their travelblogs from Internet cafes as they travel around the globe.
Many blogging tools have also been developed to improve the blogging experience, with commonly used ones providing blogrolls and feedback comment systems. Well known examples of these are blogrolling and the commenting system HaloScan. Tools such as Ecto and w.bloggar allow users to maintain their Web hosted blog without the need to be online while composing or editing posts. Enhancements to weblog technology continue to be developed, such as the TrackBack feature introduced by Movable Type in 2002 and subsequently adopted by other software companies (e.g., Userland) to enable automatic notification between websites of related content -- such as a post on a particular topic or one which responds to a post on another blog .
Blogs with features such as TrackBack are credited with complicating search engine page ranking techniques  and confusing (perhaps deliberately gaming) search engines that try to establish context.
Web hosting companies and online publications also provide blog creation tools, such as Salon, Tripod, and America Online, which calls its subscriber blogs "journals."
Often, the word blog is used to describe an online diary or journal, such as LiveJournal. The weblog format of an online diary makes it possible for users without much experience to create, format, and post entries with ease. People write their day-to-day experiences, complaints, poems, prose, illicit thoughts and more, often allowing others to contribute, fulfilling to a certain extent Tim Berners-Lee's original view of the World Wide Web as a collaborative medium. In 2001, mainstream awareness of online diaries began to increase dramatically.
Online diaries are integrated into the daily lives of many teenagers and college students, with communications between friends playing out over their blogs. Even fights may be posted in the diaries, with not-so-veiled insults of each other easily readable by all their friends, enemies, and complete strangers. Online diaries may also be a bane to job seekers, if their diaries are discovered by potential employers.
Political blogs attracted attention because of their use by two insurgent political candidates in 2003: Howard Dean and Wesley Clark. Both gained political buzz on the internet, and particularly among bloggers, before they were taken seriously by the establishment media as candidates. Joe Trippi, Dean's campaign manager, made the internet a particular focus of the campaign. Both candidates stumbled in the end, but were, at one time or another, thought of as front runners for the Democratic Nomination.
BlogPulse - Full-text search engine for blog postings. Also analyzes Top Links, Key People, Key Phrases, and produces BlogBites every day. Showcases trend graphs from the blogosphere. Allows you to create your own trend graphs.
Blogshares - A fantasy stock market where weblogs are the companies. Players invest fictional dollars on shares in blogs. Blogs are valued by their incoming links and add value to other blogs by linking to them. Prices can go up or down based on trading and the underlying value of the blog.
Weblogs.com - a list of weblogs updated in the last three hours
clarkmacleod.com A weblog focused on web design, life in Taiwan, and other random interests. http://www.clarkmacleod.com
Calamondin A combination of weblog and online journal. http://www.calamondin.com
Cabinet Cabinet of wonders, oddities, coincidences, lies, hoaxes, histories. http://cabinet.editthispage.com/
Congeries The daily web wanderings of a quirky lesbian misanthrope. http://congeries.pitas.com
Considered Harmful The ramblings and wanderings of yet another geek. http://c_harmful.pitas.com/
Continuity Stuff, randomness, and noise. The personal site of a guy in San Francisco, including a frequently updated weblog. Some assembly required. Batteries not included. http://www.continuity.nu/
Caught in Between Just another boy caught in between nonsense and non sequiturs on media, pop life and technology from a cynical brutha in Silicon Valley. http://www.lagtime.com/cib/
Chocolately Shatner Hey, I have a blog and a voicemail box. What more could I want? Oh yeah. A woman. http://www.premiumpolar.com/blog/
Catwrangler.com Idiosyncratic blog to put my compulsive writing to work. Includes travel diaries of trips to various places in the world including SE Asia, UK, Canada and US. http://www.catwrangler.com
Chaos Terminal Punk music, web development, technology in general, and life. Yum yum. http://www.bibik.org/CT/
The Coffee Cup Project Take a picture of your favorite coffee cup and send it, along with a brief description of why you love it so much. http://www.grillboy.net/coffeecup/
Consumptive Art, photography and the uncanny. http://consumptive.org/weblog/blog.html
Completing the Square Thoughts and experiences of a vegetarian college student in Atlanta. http://www.completesquare.org/
Chris Cam Intermitent stream. http://www.chriscam.com/
CollegeBlows.com Humorous rants, tales, and anecdotes from a jaded college student. http://www.collegeblows.com
Colourblinded Lots of meaningless posts about the most inane things, all written by a rather unassuming yet incredibly self-absorbed college student. http://colourblinded.tripod.com/
Cloudvark Blog Daily life in a small body piercing jewelry company. Customize yourself. http://www.cloudvark.com/blog/
Cult of Darian Weblog about life, sex, and whatever else. The site of Darian Drake. The ups, the downs. http://www.cultofdarian.com
CrazyWalker House of pure, unadulterated craziness. http://www.crazywalker.com
Clog Weblog of a london-based dutch girl. Musings and strange, funny or just bizarre links. http://www.dutchbint.org/
Chinaman's Hat a Hawaiian island off of Oahu with photos, daily thoughts, and tropical stories. Aloha. http://www.chinamanshat.com
CrustyGeek A personal weblog with art, poetry and other eclectic offerings. http://www.crustygeek.co.uk
Curmudgeonry Mutterings and grumblings about news and culture. http://www.greeblie.com/curmudgeonry/
CCCP Cultural Consumption Programme. London-based weblog on all kindsa stuff: clubs, webdesign, nastiness, politics. Occasional adult language/ themes. http://www.pressure.to/cccp/cccp.php
Crankydoodle Almost certain he's not cranky at you. http://www.crankydoodle.com/
Crumbs on the Tongue Blog about web design, content management, wine, eating, travel and the mishaps of ordinary life. http://octafish.blogspot.com/
Code: the Web Socket The ramblings of an old critical care nurse, including stories from the bedside, news links, opinions, recipes and politics. http://www.ahawkins.org/weblog.php
Central Site for the Ron Martin Fan Club Drives the ultimate uselessness of the Internet to an all time high while giving readers a chance to waste considerable amounts of valuable time. http://www.deepthought.org/~rmartin/
Calicogirl.com Observations on life after education and topics in general. http://www.calicogirl.com/
Cheap Thoughts by Ken Observations on life, history, and science with a Seinfeld-like perspective and a touch of George Costanza's cheapness. http://cheapthoughts.blogspot.com
Cyn's Sim A personal look into a computer engineering student's life at the university. Come see me, and see what life in Washington State is all about. http://www.cynsim.com
Crazy Badtz Maru Personal journal/ blog about life, girls, music, movies, anime, virginia tech, sex, zak, sumo monky and sumo monkey, friends, reno, ju jitsu, frisbee and crazy badtz maru. http://filebox.vt.edu/users/zellswor/index.shtml
Ctrl alt del From the creators of spic 'n' span comes this delightful dark comedy about life, love and the pursuit of happiness, Generation X-style. http://www.geocities.com/solar_happyface/
Cynics Tea Party Daily thoughts, random commentia, and stray neuron firings from the laughing muse. Caffeine, design, satire, theosophy, cynicism, software, cats. And anything else not nailed down. http://www.cynicsteaparty.com/
Caterina.net Science fiction writer living in Vancouver, British Columbia. An erstwhile design martinet and/ or flunky, currently on sabbatical. http://www.caterina.net
Cup O' Ambition Just a rung on the Boss-Man's Ladder. http://9to5.blogspot.com/
Calapitter Tried to tell her dreams, more bad metaphors would spill. Tried to know her heart, similes and grammar would fail. http://www.calapitter.net
Consequently An annotated bookmarks page, a scratchpad for thoughts and ideas, a place to post photos and somewhere to experiment with web design. To keep in touch with friends far away. http://consequently.org/
Cheesedip.com Occasionally cranky commentary from Manila. http://cheesedip.com
Cutie Blog Personal weblog kept by students. http://cutieblog.blogspot.com/
Crispyhead Assortment of various musings from a possible alien. http://www.crispyhead.org
Caoine Like a gingerbread skyscraper. Rich in nutrients. http://caoine.org/
Cat in the Mist Cat's personal weblog. http://www.xanga.com/home.asp?user=CatInTheMist
Cinediva.com The life, times, and exercises in randomness of a girl who likes films. http://www.cinediva.com/
Corkys Webcams Log A site for everyone who owns a webcam updates for webcams and new sites. http://www.xanga.com/corky
Chris Holstrom's Blog A personal webpage concentrating on music, literataure, film, politics, and what every other blog is about. Reviews of books, albums, and films. http://www.cholstro.net
Couture A personal blog and content based around her love of fashion. http://couture.sassybella.com/
Cockeyed Blog belongs to a frustrated common worker seeking retribution for his suffering. http://cockeyed.blogspot.com
Curb Your Dogma A 24 year attempts to justify a life altering decision. http://curbeddogma.tripod.com
Chandra K. Clarke Clean, thoughtful week humor by Chandra Clarke. Freelance writer, author, humour columnist and space enthusiast. http://chandraclarke.blogspot.com/
Curious Frog A repository for website links, opinions, observations and the occasional rant. http://www.curiousfrog.com/
Curmudgeonly & Skeptical Thirty years of public schools, and Dan Rather, have taken a toll. http://rschultz.blogspot.com/
Capricious.org A weblog in Boston with photos, stories, links and other tidbits. The life and times of Sooz. http://www.capricious.org
The Communist Squirrel Taking over the world, one treetop at a time. A weblog by Zeke Runyon. http://communistsquirrel.com/
Chris's Journal A comical look at the days events of a bloody student. http://www22.brinkster.com/regopsp22/view.asp?category=journal
TheChainGuardian Joe Adelaars' personal weblog, formerly known as Heffalump. http://adelaars.manilasites.com/
Chunshek.com Weblog and daily rambling of a Hong Kong native studying in Massachusetts who aspires to become a world traveller. Chun-shek speaks five languages and writes both poems and programs. http://chunshek.com/
Cynical Rantings Superfluous thoughts from a regular, yet potentially fascinating gal. Humor, sarcasm, observations, life from a perspective. http://cynicalrantings.blogspot.com
Cup of Chicha Cultural commentary, diary, and literary links. Nathalie is a short story writer and currently a student at the Iowa Writers' Workshop. http://chicha.blogspot.com
Crackerass Escape from your own problems and fall into rantings, anarchy, and loads of uselss crap. http://www.angelfire.com/ego/americanzero
Chronicles of Hectica A blog by Paul Simpson. General commentary, useful links, and general rantings. http://www.hectica.us
I am a Cheeseburger Personal weblog of Matthew Frederick Davis Hemming, also known as CheeseburgerBrown. http://mfdh.blogspot.com
The Calliope Project One girl's trekkings through that thing called life. http://xacal.tripod.com/cal/
Cubicle Dweller Creative writing, pop culture, geek culture, theatre, acting, and current issues. http://www.cubicledweller.ca
Contemplation 4.0.1 Welcome to my thoughts. My website also has southpark, rachael leigh cook, game faqs, cheats, walkthroughs, editors, and patches. http://contemplation401.tripod.com
Captain Kaya Styn Daily journal and humor. Comedic interpretation of current events. Original writing and unique observations. http://www.geocities.com/captainkayastyn/
Colin Crawfords Blog This is the Weblog of Colin crawford, and contains information about Lotus Notes/Domino and Java development. http://blog.timebasic.com
Communicate Mark Friedlander's personal home page, here you can communicate with him and see whats on his mind. This usually includes topics like technology, entertainment, law, film, music, science, culture and design. http://scraver.blogspot.com
Clagnut Blog with a decidely web design slant, from Richard Rutter. http://www.clagnut.com/
Commonplaces Tom's new commonplace book: variegated inconsequentiality out the wahzoo, replete with rhetorical strategies for passing the time. http://tom.weblogs.com
Craftyteen Crafts, gardening, and other diy related things. http://craftyteen.com
Claudy Mok Personal weblog of a Hong Kong girl living in the Netherlands. http://www.claudy.org/blog/
The Clumsy Twirler Diaries Personal journal of Andrew Dupuy, Louisiana swamp rat and bayou boy. http://www.clumsytwirler.com/cgi-bin/journal.cgi
The Chang Journal Personal weblog of George Chang. New England, fishing, and other interests. http://www.changnet.com/
Citizen: Arcane Basically a personal log of adventures and such as told through the eyes and fingers of a 29 y.o. Detroiter with too much time on his hands at work. http://www.citizenarcane.ca
Cohabitation Nation News and commentary about cohabitation, marriage, and other topics of interest. http://www.cohabitationnation.com/
CreepyThing.com Includes essays about Ohio football, the 9-11 disaster, garbage pail kids cards, and paranormal experiences. http://www.netwalk.com/~jpayton/
Common Monkeyflower Murph's rantings, focusing on such things as urban planning (especially transportation), world events, anarchism, computers, and whatever happens to be irritating him most at the moment. http://www.commonmonkeyflower.net/cmf.html
Czeltic Girl Commentary on her personal life and visited links. http://www.czelticgirl.com/
Crashing Silence Personal site of a 20- something BBW with a blog and cam (no nudity). Some size acceptance information and links, personal journey and other interests, food, film, photgraphy and music. http://www.crashingsilence.net
Chiefwiggles Home from Iraq. One man can make a difference. http://chiefwiggles.blog-city.com/index.cfm
CoolB.com Blog, links to favorite sites, among other things. http://www.coolb.com
Crazed Monkey Ian Stevens' personal weblog with articles on Java, XML, music, books, world politics as well as news special to Toronto, Ontario, Canada. http://crazedmonkey.com/blog/
Creative Generalist Blog and community for curious divergent thinkers who appreciate new ideas and the wide mix of sources needed to generate them. http://creativegeneralist.blogspot.com
Cambear.com A little weblog maintained by a cute little bear. http://www.cambear.com
Confused Enlightenment Thoughts and images collide to create meaningless words of infinite importance. http://www.itismylife.com
Coco Loco Christopher Taylor's editorials on science, technology, and salsa dancing. http://radio.weblogs.com/0107035/
Cindy's Blog A personal weblog by a Muslim woman living in Canada. http://pseudonymous.blogspot.com
Codshit.com Thoughts, musings, nonsense and, very occasionally, a hint of genius. http://codshit.blogspot.com/
The Chump Cool stuff submitted from cool people around the world. http://www.thechump.com
Chez Nathalie For Gen-Xers, moms, parents providing views on life, from health, diet, finance and everyday news. http://www.cheznathalie.com/
Common Monkeyflower A computer geek turned urban planning geek comments, polemically, on both fields and on whatever else comes to mind. http://www.commonmonkeyflower.net/cms/index.php
Cat Walk Feral cats and political honesty are both elusive. The first overbreeds, the second appears to be a threatened species. http://www.catoutloud.blogspot.com
Cyberia Project Fighting ignorance with every word. http://www.cyproject.net
Cory's Webstravaganza A news and rumors site. Posts links to interesting topics and gives two cents on them. Advice column and features commentary on movies, music, wrestling. http://members.shaw.ca/coryweb
Consideropsium Essentially a commentary on personal life, current affairs, war, Iraq, government, consumerism, people, Australia and anything else that takes fancy, or not. http://denizengt.modblog.com
ChrisBrunner.com The personal site of Chris Brunner including news, thoughts, links, and random notes to himself. http://www.chrisbrunner.com/myblog/
Cio Seven Cliff is original. Not. Personal website, devoid of anything of real worth. Brief updates, blogs, every so often. Some pictures and a whole section of vodka. http://www.cliff.clockerz.net/
Chez Chrissie Home page and blog of Christine Harper, 30-something writer in the West Midlands, UK. http://www.chezchrissie.co.uk
Cyber Satan Weblog, musings, philosophy and humour. http://www.cyber-satan.com