ADA vs Section 508
ADA stands for Americans with Disabilities Act. It is a broad civil rights law that prohibits discrimination against people with disabilities in Employment, Architectural Design, Transportation, Examinations & Courses, and other services offering "Public Accommodation." Even though this Act does not mention web accessibility explicitly, many of the web accessibility lawsuits in the United States invoke it as a basis for the legal complaint, because of "Public Accommodation" references. It was passed in 1990 to ensure that people with disabilities have the same rights and opportunities as everyone else.
Section 508 refers to a section in Rehabilitation Act which is a federal law. The Rehabilitation Act mandates that federal agencies develop, acquire, maintain, and use Information and Communications Technology that people with disabilities can access. The law does not solely focus on federal employees with disabilities. It also applies to people with disabilities that use federal agency resources. Rehabilitation Act was enacted in 1988 to eradicate technological barriers in information technology so that people with disabilities would have the same rights and opportunities as everyone else.