MUDs first appeared in 1978, and their popularity escalated in the USA during the 1980s, when (relatively speaking) cheap, at-home personal computers with 300 to 2400 baud modems enabled role players to log into multi-line BBSes. Roguelike games were also becoming popular at that time.
In Europe at around the same time, MUD development was centered around academic networks, particularly at the University of Essex where it was played by many people, both internal and external to the University.
The MUD scene is still very much alive on the Internet, and can be accessed via standard telnet clients. Specialized MUD clients exist that give a more pleasant user experience.
The original MUDs drew their inspiration from paper-and-pencil based games such as Dungeons & Dragons; (hence their name), and the computer game Colossal Cave Adventure. The first MUD was probably created and written by Roy Trubshaw and Richard Bartle at Essex University in the UK although the book "Dungeon Master" by William Dear, and some other sources suggest there were earlier MUD games that the Essex authors never knew about.
A version of this first MUD is still running at www.british-legends.com and a version of its descendant MUD2 runs at www.mud2.com. The first popular MUD was AberMUD written by Alan Cox, also known as Anarchy, named after the University of Wales Aberystwyth. Over time variants have diversified into other models while retaining the textual format. For example, some variants are called MUCKs, MUSHs, LPMUDs, and MOOs.
A MUSH is often said to mean multi-player shared hallucination. MUSHes descend from the program TinyMUD. MUSHes date back to the early 1990s. They are more directly concerned with role-playing (acting) than MUDs, dispensing with the experience systems. Instead, players focus on creating their character's life as accurately as possible. Members of the MUSH family include PernMUSH, PennMUSH, TinyMUSH, TinyMUSE and TinyMUX.
A MUCK, which is an acronym of multi-user chat kingdom, is similar to a MUSH in that the emphasis is on player interaction, rather than action and questing. MUCKs and MUSHes differ from IRC as a chat medium in that they provide a world, character descriptions etc in order to flesh out role-playing chat.
A MUVE is a fairly recent term which is the acronym of multi-user virtual environment. Its goal is to simply have a less narrow or ambiguous acronym for the genre.
Other variants emphasize building by providing players with a powerful programming language (as in MOOs) to make their own objects and rooms, or function as elaborate chat systems with no fantasy trappings.
When referring to MUDs, MUCKs, MUSHes etc. all alike, the term MU* is often used.
A lesser known variant is the talker, typically based on ew-too, summink, sensi-summink, playground, and plenty of other code bases. The talker is essentially a MUD, with most of the complex bits of code stripped away, leaving just the communication level commands -- hence the name talker. People who use these tend to be called spods.
The spod tends to be something of a long term fanatic. Where many mudders may move on after a year or two, people who use talkers typically have been doing so for a decade or more. Talkers are signifigantly easier to run than an average MUD, since they don't incorporate very much artificial intelligence, and they are usually much more user friendly, since there is not often much fighting as a focus. In other words, whole families of husbands, wives, children, and siblings have been known to spod in certain circles.
They also use very little network traffic, and use simple protocols, making them ideal for setting up quietly at work. Talker applications predate MUDs by many years, although some of the early ones were used to play Dungeons & Dragons; over computer networks.
The Center for Imaginary Environments Supports reality modeling technology and multi-user environments. Members are active in the development of mud software using the LPC and Java languages. http://www.imaginary.com
MUSE Ltd. Purveyors of fine, online games since 1985; principal products are MUD2 and the MUDDLE programming language and development system, both available for commercial licensing. http://www.mud.co.uk/
Richard A. Bartle: Players Who Suit MUDs Discusses whether MUDs are games, pass times, sports or entertainments and suggests four kinds of player: achiever, explorer, socializer or killer. http://www.mud.co.uk/richard/hcds.htm
Intermud Protocols An overview of the protocols that connect different MUDs together, providing intermud chat-channels and other information. http://mud.stack.nl/intermud/
An Atlas of Cyberspaces - MUDs and Virtual Worlds An atlas of maps and graphic representations of the geographies of the new electronic territories of the Internet, WWW and other emerging Cyberspaces. http://www.cybergeography.org/atlas/muds_vw.html
MUDdy Waters Detailed information about the MUDding community. Features reviews, resources and message boards. http://www.suite101.com/welcome.cfm/text_games
The MUD2 Archives Collected stories, poetry and memories from the VAX and later incarnations of MUD2. http://www.dragon-media.net/game/home_game.htm
Vae Victus Contains news, logs, history, and a message board. http://clan_vaevictus.tripod.com/news.html
Code begets community Information about Daniel Pargman's Ph.D. thesis on the social and technical aspects of managing a virtual community, a study of a Swedish MUD. Sample chapters in PDF format. http://esplanaden.lysator.liu.se/svmud/pargman/
DikuMUD Family Tree Listing of MUD codebases by heirachy for Diku and all code bases. http://www.game.org/heirarchy.html
Journal of Virtual Environments Academic journal devoted to MUDs and their uses. Formerly known as the Journal of MUD Research. http://www.brandeis.edu/pubs/jove/
Help build the largest human-edited directory on the web.