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.