Vitural Machine?

I have long wondered about this...what is a VM? My friend explained it as a computer within a computer yet it isn't a computer:confused So I figured I'd better come here!! Is it something you 'need?' What does it do, or supposed to do? I'm still using Win2000Pro (SP4) and can it, whatever it is, be used on this old of an OS? Thanks for any replies!! Oh yeah, once I find what it does, and I decide I could use it, where do you get it?

 

Mark, thanks for the reply and the links. However, I'm still pretty much in the dark because I really didn't understand that! What can I say?....'I'm not the brightest bulb in the box':-D Still, I run on the philosphy that if it isn't broke...don't fix it and since this old machine is running quite well, I think I'll forego the presence of a VM!! Again, thank very much for your time in finding those links and posting them. It was much apreicated