A "BETA" version is a pre-release - a working example of a program but not in it's final version. Generally distributed to allow people to test it for bugs, as the developer can then iron these out before it's officially released.
A Beta software is pre-release software released by many companies, such as Microsoft etc.
Companies release Beta's software to some members of the public (or usually this is limited to their Beta Testers) so they get a chance to evaluate, and try out the software before it is released to the rest of the public.
Usually when people sign up to Beta test software the have to agree to the companies Terms and Conditions. i.e. The Developer of the software expects feedback from the tester, so when users find problems and other bugs, or has any suggestions., the program can be fixed or changed before its official release.
BEWARE IF YOU ARE GOING TO INSTALL BETA SOFTWARE, some of the bugs may range in severity from minor features that don't work to problems that cause your computer to crash