WCAG is the official Web Content Accessibility Guidelines. These guidelines have been developed by a group called Web Accessibility Initiative (WAI) in W3C. The guidelines are internationally recognized and are used in policy and best practice worldwide.
WCAG has four main areas: Perceivable, operable, understandable, and robust.
Within each of these four areas there is a set of guidelines and each guideline has a set of success criteria. The individual success criteria are most commonly used for conformance requirements. They are divided into three levels: level A, level AA and level AAA. Most organizations are required to conform with level AA (all criteria on level A and level AA).
It will be very difficult for a website to conform to all criteria on level AAA. For this reason the recommendation is, that one picks from these criteria based on priority pages and target groups.