I need a Simple Defining to Virtual memory

Virtual means, 'almost', or 'not quite' or quasi. So, virtual memory is not 'real' memory but something that pretends to be real memory. When the computer runs low on real memory it uses the hard drive to dump it contents onto and pretends it's memory. So virtual memory on a computer, is any other type of storage 'other' than real memory. This is uaually called the Swapfile or Pagefile.

It is the memory provided by your RAM. (It'll likely be 512mb or 1024mb). It stores data for programs you are running, like word documents, without writing it to the hard drive. This is because it is much, much faster than the hard drive, which would be too slow. When you save the word document, it will write it to the hard drive, but before this (and whenever you have it open) it exists only in virtual memory which cannot hold it's state like the hard drive, ie. It will be wiped if the power is cut.

when you run low of real memory the memory dumps to a page file on disk which is virtual memory. virtual memory is much slower than RAM, but is faster than reading the information from it's original disk location.

a space on harddiskwindows manages it

One or more spaces of the hard drives can be used as a temporary place to store coming data when computer RAM is full, thus these parts form which are called "Virtual Memory".Virtual memory is just the space of hard drives which is used as memory (RAM).