Beta is one of the Software Development life cycle Stage. Software Develpoments go through several stages -

Pre-Alpha

This is just an abstract idea of proposed software system. Though coding start from this phase

Alpha

In this phase you will have working script but it may have lots of bugs and all the features supposed to have may not be there

Beta 

Beta is considered as a final product. In this stage the Software surely have all the intended features and are working  and the product is publicly availabe for testing. Yet this is not called final products because there may be chance of encountering bugs that may not have shown in Alpha testing.

Final Release

 once the Beta testing is successfull, final product is released.