Accessibility is the process of making electronic materials perceivable and usable by people with disabilities. Materials (including documents, videos, websites, discussion forums, etc.) are accessibility compliant when they have been structured according to widely recognized specifications.
For web pages, accessibility applies to the page structures, the content, the visual display of the content, and the way users interact with the content.
Accessibility Technology (AT) are tools like screen readers, refreshable braille devices, joysticks, blowsticks, even touch screens and keyboards.