May be this one can help us to understand what virtual memory is :

Virtual memory is a common part of most operating systems on desktop computers. It has become so common because it provides a big benefit for users at a very low cost.

In this edition of HowStuffWorks, you will learn exactly what virtual memory is, what your computer uses it for and how to configure it on your own machine to achieve optimal performance.

http://www.howstuffworks.com/virtual-memory.htm