It is software that emulates a PC computer on a Mac, so that Macs can run PC OSs and programs. I think Microsoft sued connectix for it, and connectix has had it for GOD KNOWS HOW LONG.
I hated that program! It was an unstable piece of cr@p and it was more stable when Connectix was making it! (Still sukkz though)
It's a computer emulation program(as ATI said) there is a Mac version but it was(I believe) originaly developed so PC users could emulate a virtual PC on their PC to run x86 OS's.