[ Thanks to Ian MacGregor for this
link. ]
“A user agent is a client application implementing a
network protocol used in communications within a client-server
distributed computing system. The term most notably refers to
applications that access the World Wide Web, but other systems,
such as the Session Initiation Protocol (SIP), use the term user
agent to refer to both end points of a communications session.“Web user agents range from web browsers to search engine
crawlers (spiders), as well as mobile phones, screen readers and
braille browsers used by people with disabilities. When a user
agent operates, it typically identifies itself, its application
type, operating system, software vendor, or software revision, by
submitting a characteristic identification string to its operating
peer. In the HTTP and SIP protocols, this is transmitted in a
header field User-Agent.”