Consider your audience... they may not be able to see, hear or move, they may have a text only browser or and older browser. Accessibility standards are designed to make web sites usable for these people and its increasingly becomeing important that Web designers and developers take note of their requirements. In fact in certain countries its a legal requirement whilst in some others there are legal precedence for the requirement of accessible web sites.
Today there are two primary standards that are used as benchmarks for Web Accessibility.
WCAG is produced and maintains by the World Wide Web consortium (W3C). WCAG's standards are based on three levels named Priority 1, Priority 2 and Priority 3. Priority 1 maintains the most basic requirements that a site should attain in prder to be accessible. A good place to start in getting more information about WCAg can be found in their Web Content Accessibility Guidelines 1.0
Section 508 is a US federal law enacted in 1998. Its basis was for use in all government agencies but it has since been adopted but the wider community as a benchmark for web accessibility. For more information on Section 508 we suggest you have a look at the official website