Virtual machine - Wikipedia, the free encyclopedia
Long story short; You run an OS inside an OS
Example:
You're a Mac user, You can use a VM to run a Windows OS or Linux
You're a Linux user, You can use VM to run Windows or Mac
If you're a standard typical user; the odds are, you'll never need to use VM; As you can just bot on your PC...
You turn your PC off, all programs are stopped/closed, including VM.
regards
Tw
P.s - Easiest way to explain it maybe is like running a program as admin in Vista/W7, You simply run the program through a OS emulator