From Microsoft:
The Windows 7 Beta is a pre-release version of Windows 7, our next client operating system.
Beta software is for testing so if you like testing software, we hope you’ll try it out. By installing the Windows 7 Beta on your PC, you help our engineers find and fix bugs in the code before we deliver the final version.
It takes hundreds of Microsoft engineers around the world to create each new release of Windows—but only you can help us perfect it.