Standards
The World Wide Web Consortium (W3C) is an independent body which establishes recommendations known as "Web Standards" which are designed in essence to try to ensure that we can all enjoy the web regardless of the technology we're using and regardless of any disability we may have.
These Standards help me to make websites that are accessible to more people, quick to down–load, more Search Engine–friendly and easier to maintain. In short, abiding by Web Standards is a Good Thing TM
Accessibility
The following is, according to the Web Accessibility Initiative, the definition of Accessiblity when used with reference to websites:
Web accessibility means that people with disabilities can use the Web. More specifically, Web accessibility means that people with disabilities can perceive, understand, navigate, and interact with the Web, and that they can contribute to the Web. Web accessibility also benefits others, including older people with changing abilities due to aging.
It should be borne in mind that although making your website accessible is both obviously a laudable goal in itself and a sensible business decision, it is also a legal requirement in the UK and many other countries.
The Disability Discrimination Act (1995) 
states that it is the duty of companies to ensure they make "reasonable adjustments" to ensure access to facilities, goods and services by disabled persons. It also gives the example of a service as an Airline selling tickets on–line which clearly indicates it's relevance to websites.
I personally like to take a slightly broader view of the term "Accessible", with the ultimate aim of anyone, anywhere being able to view my sites on whatever technology they have to hand. All my sites are now built with accessibility in mind from the beginning. I don't charge extra for this – it's the only way I build websites.