For those unfamiliar with accessibility issues pertaining to Web page design, consider that many users may be operating in contexts very different from your own:

They may not be able to see, hear, move, or may not be able to process some types of information easily or at all.
They may have difficulty reading or comprehending text.
They may not have or be able to use a keyboard or mouse.
They may have a text-only screen, a small screen, or a slow Internet connection.
They may not speak or understand fluently the language in which the document is written.
They may be in a situation where their eyes, ears, or hands are busy or interfered with (e.g., driving to work, working in a loud environment, etc.).
They may have an early version of a browser, a different browser entirely, a voice browser, or a different operating system.

Defination :
Accessibility refers to ensuring that Content is accessible, ie. ensuring that Content can be navigated and read by everyone, regardless of location, experience, or the type of computer technology used. Accessibility is most commonly discussed in relation to people with disabilities, because this group are most likely to be disadvantaged if the principles of accessible Web design are not implemented. Failure to follow these principles can make it difficult or impossible for people with disabilities to access Content. Creating accessible Content should be an integral part of the Web design philosophy, and accessibility features should be incorporated into all aspects of the design process. Testing for accessibility should also be incorporated into any and all user testing regimes, and should never be seen as an isolated event that can occur after other user testing has taken place. Designing for accessibility is thus as much a strategic issue as a purely technical one

W3- Web Accessibility Initiative (WAI) :
These guidelines explain how to make Web content accessible to people with disabilities. The guidelines are intended for all Web content developers (page authors and site designers) and for developers of authoring tools. The primary goal of these guidelines is to promote accessibility

Section 508 Standards
In 1998, US Congress amended the Rehabilitation Act to require Federal agencies to make their electronic and information technology accessible to people with disabilities. Inaccessible technology interferes with an individual's ability to obtain and use information quickly and easily. Section 508 was enacted to eliminate barriers in information technology, to make available new opportunities for people with disabilities, and to encourage development of technologies that will help achieve these goals. The law applies to all Federal agencies when they develop, procure, maintain, or use electronic and information technology.