User Agent

HTTP request param sent by Web Browser

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.43

some Web Server-s intentionally deliver different pages based on this param - often due to incompatibilities across browsers, esp with DHTML, JavaScript, etc. (in 2005 this shouldn't be an issue anymore, but there are still plenty of Banks etc using MsIE-specific code).

http://www.whatsmyuseragent.com