If you go to W3C you will see the complete works on Web Site accessibility.
Its a communications fundamentally designed for all people. It’s mission is to lead the web in the direction of those users with disabilities, i.e. hearing, movement and sight etc.
The web removes barriers to communications and Interaction that a lot of people face in the physical world. Badly made web sites, technologies and web tools can create barriers that exclude people from using the web.
Most organisation already have a web site, and most of those sites i bet gave no thought to accessibility. Accessibility barriers make it difficult for some people with disabilities to use the site.
My advice is to develop a plan for web site accessibility, for example, if your starting web accessibility for a new site, many issues you will come across will apply here at WAI.