Web Accessibility Initiative - Accessible Rich Internet Applications (often referred to as just ARIA) defines ways to make web content more accessible. The attributes center around: roles, properties, and states.
These attributes only affect the accessibility of your website, they don't change structure, DOM, etc.
Ideally you don't use WAI-ARIA attributes unless you have to. It's better to rely on existing semantic HTML elements where possible. However, for more complicated patterns, it's sometimes necessary. [[20220801115655-no-aria-better-than-bad-aria]]
WAI-ARIA also has 'Authoring Practices' - design patterns made by W3C to help implement common patterns accessibly with WAI-ARIA.
The Web Content Accessibility Guidelines are a standard for accessible web content. It's based off four primary principles: perceivable, operable, understandable, and robust.
It contains conformance levels, as it's often an important legal requirement for websites to have a standard level of compliance. The levels range from A, to AAA. Note that the general goal for most websites to achieve is AA.
WCAG is more of a standard, and WAI-ARIA is more of a toolset and methods. This means that it makes far more sense to be WCAG-compliant than WAI-ARIA compliant.
WAI-ARIA often focuses more on dynamic content, while WCAG focuses on static content.
Since WAI-ARIA also contains rules on how roles, properties, and states should be used, WCAG has rules pertaining to the use of WAI-ARIA. For example, WCAG 1.3.1 is a rule around WAI-ARIA roles.