According to the User Agent Accessibility Guidelines (UAAG) 2.0:
A user agent is any software that retrieves, renders and facilitates end-user interaction with web content. User agents include web browsers, media players, add-ons (plug-ins & extensions), and web applications that help in retrieving, rendering and interacting with web content.
In order for web content to be accessible, web content developers and designers must create accessible content, and user agents must be able to deliver that content in an accessible way. The W3C, through its Web Accessibility Initiative (WAI), has developed guidelines for both parties. Guidelines for content developers and designers are the Web Content Accessibility Guidelines (WCAG) 2.0. Guidelines for user agents are the User Agent Accessibility Guidelines (UAAG) 2.0. A user agent that conforms to the UAAG has an accessible user interface and is able to able to communicate effectively with other technologies (especially assistive technologies). Furthermore, all users, not just users with disabilities, are expected to find conforming user agents to be more usable.
Note that in addition to the WCAG and UAAG, the W3C has created a third component that is necessary for web accessibility: The Authoring Tool Accessibility Guidelines (ATAG) 2.0. For details about how all three components work together, see Essential Components of Web Accessibility.
The following Knowledge Base articles may also be of interest: