An accessible web site is a web site that can be used by disabled users. Web standards make it easier for people with disabilities to use the Web.
Blind people can use computers that read Web pages for them. People with poor sight can rearrange and magnify standard Web pages.
The WAI (Web Accessibility Initiative) is a set of guidelines intended for web developers, authors, and designers- about how to make the web content accessible to people with disabilities.
Here is a brief summary
More information can be found at w3Schools including reasons for web developers to make web sites accessible.
After searching the net for information on accessibility I found a Web Site Accessibility Testing Tool
I also found a blog commited to raising the awareness of web site accessibility and marketing containing lots of useful information.
It’s suitably named Web Site Accessibility Blog