In computing, a User agent is software (a software agent) that is acting on behalf of a user. For example, an email reader is a Mail User Agent, and in the Session Initiation Protocol (SIP), the term user agent refers to both end points of a communications session.
The Hypertext Transfer Protocol identifies the client software originating the request, using a “User-Agent” header, even when the client is not operated by a user.
In HTTP, the User agent string is often used for content negotiation, where the origin server selects suitable content or operating parameters for the response. For example, the User agent string might be used by a web server to choose variants based on the known capabilities of a particular version of client software.
Some day ago I need to understood wich device view a website and the only way to do it is to check the user agent string. This website made it possible because of its list.