Virtual PC is a virtualization program that simulates a hardware environment using software. It is currently developed by Microsoft and intended for use on Windows.
Virtual PC is software that hosts virtual machines (which use software to simulate real hardware). It is designed for the Windows operating system, and is currently developed by Microsoft.
For Windows 7, there is a special version of Virtual PC called "Windows XP Mode" that can be used to run legacy software designed for Windows XP in a special compatibility environment. It is available free of charge for customers with qualifying versions of Windows 7.
Older versions of Virtual PC were released for the Mac OS, and allowed it to run Windows-based software under an virtualized Windows installation. They were chiefly developed and maintained by Connectix, before the software was acquired by Microsoft.
More information about the history and implementation of Virtual PC is available on Wikipedia.