hey could anybody here give me a simple description of what a virtual machine actuall is...i jst cant get it:-/ need some help....:S

A virtual machine is "as if" a real machine, but running as a program on some other machine. This is useful in a variety of ways. Two of them are:

  1. You can run several virtual machines (even different OSs) at the same time on one piece of hardware.
  2. You can use a virtual machine that has no network connections as a test bed for a suspect applicaation.

hey griswolf thanks very much for ua reply......can u tell me ...does a virtual machine has its own operating system....and how it works in kernel mode...????

You have to install an operating system in the virtual machine software. Its just like installing an OS after you format your hard drive.
Some gud VM softwares are- Sun Virtual box (open source) and VM ware (paid)

Very broadly (and therefore wrong in detail), it works by time slicing the actual hardware and apportioning those slices to each of the virtual machines. Each virtual machine is as if running on its own hardware, so as titan5 points out, each one needs an OS to be installed before it is of much use.