What is my User Agent? This page displays your User Agent String and Screen Resolution. This is a very basic output of the User Agent also referred to as simply UA. Your User Agent tells every site your visit what browser you're using. Sites are able to detect your UA and redirect to a more fitting page for your browser if desired. In other words, if you go to a site with this detection in place from a mobile phone the site might detect you are a mobile user via the UA and redirect you to their mobile site if available. This doesn't work on all sites becuase not all sites have UA detection in place. We have it built in to WhatIsMyIP.com. We detect most mobile phones and redirect them to our mobile page however, if you go to WhatIsMyIP.com from a desktop computer, you'll get redirected to the What Is My IP homepage.
UAs also tell the sites you're visiting some browser related software which you may have installed. For example, this UA (Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0; Trident/4.0; GTB6; Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1) ; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; InfoPath.2; .NET CLR 3.5.21022; .NET CLR 1.1.4322; .NET CLR 3.5.30729; OfficeLiveConnector.1.3; OfficeLivePatch.0.0;) tells us that the visitor is using Internet Explorer 8, Windows Vista, has Media Center PC 5.0 installed, Office Live Connector, etc. So if we had a special section for Vista users, we could redirect this person based on the Windows NT 6.0 string to a sub section in our site related strictly to Vista. This User Agent on the other hand, Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.1.3) Gecko/20090824 Firefox/3.5.3 tells us the visitor is using FireFox 3.5.3 and Windows Vista. Not much else.
Having the knowledge from above, User Agent Spoofing is used by many programmers that don't want people to know how they're getting to the site(s). Some programmers will find legitimate UAs and program their script to use such UAs. UAs can be blank, or contain custom information like a URL to a site that might promote a particular browser or script.