The legislation referred to as "Section 508" is actually an amendment to the Workforce Rehabilitation Act of 1973. The amendment was signed into law by President Clinton on August 7, 1998. Section 508 requires that electronic and information technology that is developed by or purchased by the Federal Agencies be accessible by people with disabilities. (excerpt from jimthatcher.com)
Web Content Accessibility Guidelines (WCAG) is developed through the W3C process in cooperation with individuals and organizations around the world, with a goal of proving a single shared standard for web content accessibility that meets the needs of individuals, organizations, and governments internationally. (excerpt from w3.org)