From Joomla! Documentation
There are two primary standards that are used as benchmarks for Web Accessibility.
Web Content Accessibility Guidelines (WCAG)
The Web Content Accessibility Guidelines are produced and maintained by the World Wide Web consortium (W3C). WCAG's standards are based on four Principles: Perceivable, Operable, Understandable and Robust. Get more information about the Web Content Accessibility Guidelines 2.0 See also the Introduction to Web Accessibility.
Section 508 is a US federal law enacted in 1998. Its basis was for use in all government agencies but it has since been adopted by the wider community as a benchmark for web accessibility. For more information on Section 508, see the the official website.