Basically, technology is accessible if it can be used as effectively by people with disabilities as by those without. This doesn't mean that a blind user will complete a task on a web site as fast as one who can see. Listening to a screen reader takes longer than looking at the screen and reading. But the processes must be comparable. How can that definition of "accessible" help a web developer know what to do to make his or her product comply with Section 508? Truthfully, that definition isn't helpful. Developers don't know how people with disabilities use computers. What people? What disabilities? Companies don't have the resources to have all their products tested by people with all disabilities; it is very good to do usability testing with people with disabilities, but hopeless to use such testing as the measure of product accessibility.
Instead, Section 508 required that the Access Board define accessibility through a set of standards. To do that the Board empanelled the Electronic and Information Technology Access Advisory Committee (EITAAC) in October of 1998.
EITAAC was composed of representatives from industry, academics, government and disability advocacy organizations. The group proposed standards for accessible electronic and information technology. The Access Board took the recommendations of the advisory committee and created a "proposed rule" issued by the Access Board on March 31, 2000. After a public comment period a final rule was issued on December 21, 2000. This final rule contains the standards for accessible electronic and information technology that are the heart of the Section 508 enforcement which began in June of 2001.