Web accessibility means that people with disabilities can access the Web and its contents, no matter the nature of the disability (physical, intelectual or technical) or if it derives from the context of use (technological or environmental). This quality is closely related to usability.
When the websites are designed to be accessible, all the users can access in equal conditions to the contents. For instance, when a site has a XHTML code semantically correct, it is provided an alternative equivalent text to the images and the links get a relevant name, so this allows the blind users to use screen or Braille line readers to access the contents.
When the videos have subtitles, the users with auditory disabilities will understand them perfectly. If the contents are written in a simple language and illustrated with diagrams and animations, the users with dyslexia or learning problems will be in better conditions to understand them.
If the text is big enough, the users with visual disabilities will be able to read it without difficulties. Also, an adequate size of the buttons or active areas will make easier its use to those users that cannot control the mouse precisely. If actions that depend on a concrete device are avoided (press a key, click with the mouse), the user will be free to choose the device that better suits him/her.