What is a Screen Reader?

http://www.nomensa.com/resources/articles/accessibility-articles/what-is-a-screen-reader.html

A screen reader is a software application that enables people with severe visual impairments to use a computer.
Screen readers work closely with the computer’s Operating System (OS) to provide information about icons, menus, dialogue boxes, files and folders.
A screen reader provides access to the entire OS that it works with, including many common applications.