There are generally three compliance levels within WCAG (Web Content Accessibility Guidelines): A, AA, and AAA. Each level comprises guidelines that must be fulfilled to consider the website available for all users.
The difference between compliance levels provides website developers with an organized system for good, minimal, and better accessibility. Besides, the different WCAG classes also deliver more flexibility, so even exceptionally complicated websites or state-of-the-art technologies can keep a minimum level of compliance.