ADA stands for the American Disabilities Act, which is "a civil rights law that prohibits discrimination against individuals with disabilities in all areas of public life, including jobs, schools, transportation, and all public and private places that are open to the general public." (https://adata.org/learn-about-ada).
WCAG stands for the Web Content Accessibility Guidelines (WCAG). These are the practical standards for the ADA – by creating a website that adheres to the best practices set in WCAG, you're helping your site be in-line with ADA standards. (https://www.w3.org/WAI/standards-guidelines/wcag/).
WCAG is not always black and white; its rules often have a variety of interpretations.