A user agent is an application which is used to browse the World Wide Web. Web user agents range from web browsers to search engine spiders, but also include screen readers and braille browsers which can be used for people with disabilities.
When internet users visit a web site, users leave, as a visit sign, an information string about their browsers. This information string is the user agent. Different web browsers and different operating systems leave different user agent strings, and some of the most common are listed below.
Some websites fail to comply to network (IETF) or Web (W3C) standards. Sometimes it is possible to still gain (limited) access to such sites by spoofing the user agent as being Internet Explorer, for historical reasons.
Another reason web browsers allow users to cloak the real user agent is that many websites use badly written Javascript which locks out all browsers except Internet Explorer or Netscape. To combat this selective lock-out browsers like Opera and Safari by default identify themselves as another browser, but they will still add the real browser version in the user agent string (for example at the end). It should be noted that Internet Explorer itself claims to be a "compatible" version of Mozilla, i.e. Netscape.
As of 2004 most websites are more standards-compliant than they were in 1997 when the 'browser sniffing' as outlined above began, but especially on smaller non-corporate websites outdated Javascript which will still lock browsers other than MSIE or Netscape is still in use. This is due to such programmers doing voodoo programming by cutting and pasting older code, without actually thinking what this does to their website.
