Client Hyper-V® is the virtualization technology built into Windows® 8. Client Hyper-V is the same virtualization technology previously available only in Windows Server®. A similar functionality in Windows 7 is called Windows XP Mode. Client Hyper-V enables you to run more than one 32-bit or 64-bit x86 operating system at the same time on the same host computer. But instead of working directly with the computer’s hardware, the operating systems run inside a virtual machine (VM). Hyper-V enables developers and IT professionals to easily maintain multiple test environments and provides a simple mechanism to quickly switch between these environments.

There’s a great starter guide at http://www.microsoft.com/en-us/download/details.aspx?id=36188.