The Web Content Accessibility Guidelines (WCAG) is a set of guidelines created by the Web Accessibility Initiative (WAI).
Although version 2 is being created at the moment, most website authors use the version 1 guidelines in an attempt to prove their website is accessible.
Typically checks are done to "priority 2", sometimes referred to as the website being at level "AA". There is a priority 3 (AAA), but this is more considered as a level you "may" satisfy, and in some cases is impossible to achieve.
To make life easier, there are several automated accessibility checkers available, however, these should only be used as a guide... as the examples below show, its possible to create a webpage that fools these accessibility checkers.
Any feedback would be greatly appreciated, I don't include comments due to the admin time required, but if you email me, I will reply and make appropriate updates. Also, if you would like to take a copy of this article, please read the terms this article is released under. This article was originally written Sunday 5th August 2007.