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Crap. Incorrect answer
Virtual Memory is a mechanism to make every program feel that it has access to a larger addressable memory space than what actually exists on the system. On a 32-bit machine, that's 4G (2pow32). V2P mapping is done through page tables, that map Virtual memory pages (4K, usually) into actual backing physical pages.

Virtual memory concept virtualizes the main storage available to a process or task, as a contiguous address space which is unique to each running process, or virtualizes the main storage available to all processes or tasks on the system as a contiguous global address space. Whereas main memory is amount of physical memory (usually RAM) allocated to the system.

For more info:
en . wikipedia . org /wiki/Virtual_memory

Virtual memory is a place on the hard drive (a swap file) to swap programs to if there's not enough physical memory (RAM) to hold them. A program that's swapped out can't run - another program has to be swapped out to make room for that program to be swapped back in. Every time you click on a program that's swapped out, it has to get swapped back in.