Web accessibility means that people with disabilities can use the Web. More specifically, Web accessibility means that people with disabilities can perceive, understand, navigate, and interact with the Web, and that they can contribute to the Web. Web accessibility also benefits others, including older people with changing abilities due to aging.
The Americans with Disabilities Act (ADA) has been in existance for years, and has acted to allow the disabled to take a greater role in society. This is more than installing cuts in curbs that allow the wheelchair users of the world to navigate sidewalks. This is about designing everything around us with an eye to accessibility and usability for those who have different abilities than most people. While the ADA is an American law, there are probably similar laws in other countries. Additionally this is just a good and humane idea to give opportunities for more people to participate in society.
As the web grows more important and critical in society, we must consider the role of Accessibility in web sites and web technology.
Resources on Web Accessibility: List of resources for understanding Web Accessibility. The Web Accessibility Initiative has already done all the heavy lifting, and has some excellent guidelines.