A lot of people are doing a lot of things on the internet, and some of them face challenges. Some people who are legally blind use special devices to see the web. Some people who are colorblind may not be able to tell your menu choices from your background color. Just like the Americans With Disabilities Act provides for accomodation and accessible physical design for people with special needs, there are factors to consider in website design.

The Web Accessibility Initiative has developed WCAG 1.0 (Web Content Accessibility Guidelines), which is the accepted accessibility standard for websites. If your website is built to meet WAI Web Content Accessibility Standards, you can display these icons on your website:
   

Section 508 is a federal law concerning website accessibility. While it applies only to federal websites, increasingly contractors and state agencies that deal with the federal government are being asked to ensure their sites are Section 508 accessible.

In 1995, CAST developed the "Bobby" (UK slang for policeman) standard for Accessibility. The Bobby suite was purchased by IBM in 2008, and is now bundled in the IBM Rational Policy Tester Accessibility Edition. You'll still see Bobby icons on some websites:

The major takeaway is that web accessibility is not just another standard or a way to hang kludgy trophy icons on your website: designing and coding with web accessibility in mind produces better designs, better sites, better search engine results, and a better user experiece.