* What is virtual machine?

Essentially, virtual machine is a software which provides abstract computers on top of a real computer.

Nowadays, a computer usually consists of CPU, memory, hard drive, cd-rom, network card, serial/parallel ports, etc.. A programs is basically a sequence of instructions. Loading, running, and storing the results of the program involve the hardware components above.

Virtual machine can provide an abstract computer that has the abstract components, which are mapped to the real components. For example, a hard disk partition is mapped to a file in real hard drive. A cd-rom is mapped to a cd image file in real hard drive or to the real cd drive. Most of the instructions ????uted at the abstract processor is ????uted at the real processor directly for better performance. However, some instructions have been identified to have more control over the computer system . So when ????uted at the abstract processor, these instructions are intercepted by the virtual machine and simulated to ensure safety of the real computer. For example, the abstract computer ????utes the instruction to shut down the machine, the virtual machine basically shuts down the abstract computer, and the other softwares running on the real computer are never affected.

Normally, the abstract machine only has access to the data on the abstract hard drive, and has absolutley no access to the other information on the real hard drive. If exchange of information between either multiple virtual computers or with the real computer is needed, some network protocol is used.

* What can virtual machine do?

As long as your machine has enough resource, i.e. the cpu is very very fast, a lot of memory, etc. you may simultaneously run as many abstract computers as you like.

You can copy a abstract computer and run it on another machine. In another word, you may save a copy of your abstract computer and do not have to reinstall your system when it is messed up.

If you are not sure about a software or a web site, you may install it or access it from an abstract computer

* How can we use it?

You can get a trial version of VMware workstation or microsoft's Virtual PC 2004 (used to be connectix, acquired by MS in 2003).