Accessibility is about ensuring that all users can access your website, irrespective of any disabilities they may have or what technology they are using. There are a number of things you can do to make a website more accessible:
- Your website must be able to function with all the different browser technologies available
- Forms must be accessible to all users
- Users should be able to process content quickly and easily
- Structure and presentation should be separated
- The user should be able to control your website
You can find a good introduction to accessibility in the article Web accessibility - The basics on the Webcredible web site. You can also read a number of other articles about accessibility on the same site and download a white paper. (You'll need to provide an e-mail address to have the link mailed to you.)
Obviously, accessibility and usability are closely related. In practice, highly accessible web sites are also highly usable. You can find out a lot more about the links between usability and accessibility by reading the articles mentioned above.