In the broadest sense, assistive technology is any technology which enables someone to do something they otherwise couldn't. A ladder, for instance, could be considered assistive technology, since it facilitates access to otherwise unreachable spots. In the realm of computers and disability there are two general kinds of assistive technology. Adaptive technology places fairly modest demands on a computer system.
Access technology makes rather greater demands of the computer system.
Since access technology provides access to theoretically every aspect of every program running on that computer system to someone with a specific set of impairments, access technology needs to know potentially everything that is going on in the user interface of every program it is providing access to. The main point behind all these laws is that they give students, state and federal employees, consumers, and their representatives the power to strongly influence future purchasing in a diverse set of public and private markets. Section 504 of the Federal Rehabilitation Act was passed in 1973 and applies to education contracts that utilize federal funds. It was passed so that students with disabilities could be mainstreamed into the regular public education system and could begin building the skills needed to compete in the job market. It basically states that federally funded technology purchases for education must include technologies and support for use by all students.