The Web Content Accessibility Guidelines (WCAG) 2.0 is a guidelines document published by the Web Accessibility Initiative (WAI) which intends to provide web developers with a series of “testable statements” they can use in order to improve access to and usability of their website for users with disabilities. (source)
The criteria presented by the guidelines are split into four “principles,” each of which describes a specific aspect of a website’s user experience. The criteria in these principles are technology-independent, meaning the guidelines do not present specific ways of creating an accessible website in (e.g.) HTML or PHP. This theoretically means that the guidelines do not deprecate as quickly as one tied to a specific technology or markup language.
Principle one is that a website must be Perceivable:
Criteria in this principle deal with content—specifically, with ways in which a website should present its content so that users can perceive it. This includes providing text alternatives for images, audio, video, and other non-text content; making a distinction between layout and content so that users who are not on a standard computer screen can still perceive it in a coherent way; and several other things.
Principle two is that a website must be Operable:
Criteria in this principle deal with navigation. Websites must be keyboard-accessible, must provide users with enough time to process and interact with content, must not be designed in a way that causes seizures, and must help users navigate.
Principle three is that a website must be Understandable:
These criteria present ways in which end users must be able to understand a website’s content and how to use it. This includes things like specifying a language for the website, making sure pages on it operate predictably, and helping users correct mistakes.
Principle four is that a website must be Robust:
Principle four is essentially a catch-all principle which states that a website and its content must be usable on a variety of devices and with a variety of tools (“user agents”) such as assistive technologies.
Beyond these four principles, WCAG 2.0 also presents three levels of conformance: A, AA, and AAA, with A being the minimum level required for conformance to the guidelines.
The full WCAG 2.0 guidelines are free to read on the WAI website, and include more specific details about meeting the criteria, a glossary, and access to older versions of the guidelines.