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. In the Session Initiation Protocol (SIP), the term user agent refers to both end points of a communications session.

A web browser (commonly referred to as a browser) is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier (URI/URL) and may be a web page, image, video or other piece of content. Hyperlinks present in resources enable users easily to navigate their browsers to related resources.