The user agent of a web browser is the string that every browser sends as identification when requesting a web page. It contains several information about the browser and the system. For example:
Mozilla/5.0 (X11; U; Linux i586; it; rv:5.0) Gecko/20100101 Firefox/5.0
A complete list of user agent strings can be found on the http://www.useragentstring.com/ website.