Web accessibility is the practise of making websites that can be used by everybody, regardless of ability or disability.
This short answer is “everybody”. This isn’t as trite as it sounds:
The last benefit raises another important point: search engines are blind. What’s good for accessibility is typically good for search engine optimisation, descriptive link text being a good example.
Many countries have laws covering access to websites, much as they do for access to physical business premises. In many cases these laws are based—at least in part—on the Web Content Accessibility Guidelines (WCAG), a set of recommendations first published as part of the Web Accessibility Initiative back in 1999.
In United States law web accessibility is covered by Section 508 of the Rehabilitation Act. In the UK, it falls under the Disability Discrimination Act.
If considered from the outset of a project, web accessibility does not carry a big price tag. Retro-fitting accessibility to an existing web site, particularly one that hasn’t been built with web standards can be another matter entirely, as the Sydney Olympics Committee found out to their cost in 2000.