Originally Posted by TheLaughingMan
It is an Operating System installed to run within another OS kinda like a program. You have to dedicate a portion of your RAM and at least 1 CPU core to run only task for the Virtual Machine as though it were a separate computer.
Simply put, it is a software based clone of a real computer you can run along side your actual OS on your computer (if your CPU supports the feature).
and what can a Virtual Machine be used to do/achieve?