Progressive enhancement uses web technologies in a layered fashion that allows everyone to access the basic content and functionality of a web page (regardless of browser) and providing an enhanced experience to those with better bandwidth, more advanced browser features or more experience.
Progressive Enhancement is a development technique that stresses the primacy of the semantic HTML, then testing for browser-capability and conditionally "layering" on JavaScript and/or CSS enhancements for the browsers that can utilize those enhancements.
One of the keys is understanding the Progressive Enhancement technique tests what the browser can do, as opposed to browser-sniffing.
Progressive-Enhancement is inherently (section 508) accessible; it provides for meeting the letter of the law and the spirit of the rule.
The Filament Group wrote the excellent "Designing With Progressive Enhancement" book on the subject.
