The term "virtual computer" is an overused term that is used in a lot of different ways. If there is anything close to a single definition it's running a foreign operating system or programs designed for a different operating system within the system currently loaded on the machine. An example would be a program that emulates Linux on a machine running Windows.
pretty much what rob said is correct. I personally have a windows machine but I use a program called virtual box to allow me to emulate the use of linux ubuntu and I also use it when I want to run snow leopard.