WCAG organizes its requirements around four principles, often remembered by the acronym POUR: content must be Perceivable, Operable, Understandable, and Robust. Each principle breaks down into testable success criteria, grouped into three conformance levels: A (minimum), AA (the level referenced by almost every accessibility law worldwide, including the ADA, Section 508, and the European Accessibility Act), and AAA (the highest, rarely required in full).
The current version is WCAG 2.2, published in October 2023, which added nine new success criteria on top of WCAG 2.1, mostly focused on mobile accessibility, cognitive disabilities, and low vision. Most legal accessibility requirements today point to WCAG 2.1 or 2.2 Level AA as the practical target.