What is a screen reader?
A screen reader is a software application that attempts to identify and interpret what is being displayed on the computer screen and convert the text into Speech output. Screen readers are a form of assistive technology (AT) potentially useful to people who are blind, visually impaired, illiterate or learning disabled, often in combination with other assistive technologies such as screen magnifiers. More information at 
Screen readers in Wikipedia.
How does Screen readers work
On a windows platform any Active Accessibility Client (screen reader) will send a request to the active accessibility server to get the information of any UI element. A dynamic-link library (DLL), OLEACC, provides the Active Accessibility runtime and manages requests from Active Accessibility clients. The servers communicate with clients by sending event notifications and responding to client requests for access to UI elements.
Screen Reader commands & Gestures
The following table provides the list of screen readers and links for the their corresponding commands or gestures.
| Screen reader | Vender | Links for commands/ gestures |
|---|---|---|
| JAWS | Freedom Scientific | JAWS commands |
| NVDA | NV Access | NVDA Commands |
| Window Eyes | GW Micro | Window Eyes Commands |
| VoiceOver on MAC | Apple | VoiceOver for MAC commands |
| VoiceOver for IOS | Apple | VoiceOver gestures for IOS ) |
| Talkback on Android | Talkback gestures | |
| Narrator on Windows mobile | MicroSoft | Narrator gestures (Future link) |