There are 3 distinct levels that build on top of each other.
Easiest to achieve.
More comprehensive and covers all use cases for people with disabilities. They are able to perceive, operate, understand, and interact with the site. Most legal requirements have AA as the minimum, which also includes Level A standards.
This is going the extra mile. Usually features that are very specific to users with disabilities with additional enhancements. Unless the primary user audience are people with disabilities, it is not recommended to dedicate resources and budget to achieving Level AA Conformance.
Section508 preceded the Web Content Accessibility Guidelines (WCAG) standards. Once Section508 was enacted in 1998, the internet was moving at a much faster pace and the requirements were becoming outdated. A group of passionate web accessibility experts came together to develop a series and guidelines of best practices. Conformance to WCAG typically meant that Section 508 requirements were also met.
The WCAG gained mainstream adoption and the US Access Board in their years-long process of a Section508 Refresh, ultimately declared that the new 508 Requirements are to conform with WCAG 2.0, AA.
WCAG 2.1 was released in June 2018. It is a continuing evolution of growing website accessibility standards. It contains 17 new 'rules' that benefit:
Users with Cognitive and Learning Disabilities
Users with Low Vision