Accessibility commonly refers to the activity of making the web highly usable for people with disabilities.
The most important and comprehensive site discussing Web Accessibility is the W3C's Web Accessibility Initiative (WAI). Their home page has a list of Resources, including introductions, tips, FAQs, guidelines, checklists and descriptions of techniques for making sites accessible.
You may also look at Google's directory page for accessibility.
Of particular interest to Web Designers is the W3C's Web Content Accessibility Guidelines 1.0. This should be considered required reading. It describes 14 guidelines for making web content accessible:
Under each guideline are several checkpoints. There are over 60 checkpoints in all. The W3C has published a prioritized list of these checkpoints. They are ranked as follows:
The W3C's checklist is excellent: you would be wise to read all these checkpoints (there are a few dozen!) and look at the associated implementation techniques that accompany them. (This could take upwards of 20 hours to read and study properly, even for experienced web designers, but is well worth it.)
You can certify yourself that you conform to W3C Accessibility Checkpoints and put a logo on your conforming pages. You can claim Level A (Priority 1 checkpoints met), Double-A (Priority 1 and 2 met) or Triple-A (Priority 1, 2, and 3 checkpoints met). If you use these logos you are morally responsible for actually meeting the checkpoints.
Independent Certification: A group called the Accessibility Center of Excellence will certify a cite if you satisfy the W3C's Priority 1 Checkpoints. An example certified site is http://www.ca.gov/. (The fact that this center is part of a company currently in Chapter 11 should not detract from the fact that at least someone out there cares about accessibility.)
Some of Jakob Nielsen's Alertbox articles discuss accessibility. These are required reading!