Screen Readers are often tightly integrated with a computers operating system as they need to provide information about icons, menus, dialogue boxes, folders, files and documents.
There are lots of screen readers available, some are native and come with the operating system, others need to be purchased separately and as a result, may have more features. Common screen reading software includes:
JAWS has been established for many years and is one of the most popular screen readers:
Apple offer VoiceOver as part of Mac iOS:
More information about screen readers and how they work can be found at WebAim: