Alpha Testing
Alpha Testing is mostly like performing usability testing which is done by the in-house developers who developed the software or testers. Sometimes this Alpha Testing is done by the client or an outsider with the presence of developer and tester. The version release after alpha testing is called Alpha Release.

Beta Testing
Beta Testing is done by limited number of end users before delivery, the change request would be fixed if the user gives feedback or reports defect. The version release after beta testing is called beta Release.

Gamma Testing
Gamma Testing is done when the software is ready for release with specified requirements, this testing is done directly by skipping all the in-house testing activities.