The Web Accessibility Initiative (WAI) aims to provide guidelines and support materials to help better understand and implement Web accessibilty. They are generally regarded as the international standard.
Unsure why accessibilty is so important for the web? W3C-WAI will get you caught up on How People with Disabilities Use the Web, specifications in place to help such as Accessible Rich Internet Applications (ARIA), the actual standards themselves (WCAG 2.0), and how to test and evaluate your sites.