What is Virtual memory?
Virtual memory is an extension of your computer's physical memory (memory chips or RAMs). In order to be able to run multiple programs at once, the computer looks for areas of RAM that have not been used recently and copy them onto the hard disk into so called page files. This frees up space in the RAM to load the new application.
The hard drive (virtual memory) is much slower than actual memory chips. When you don't have enough RAM, the operating system has to constantly swap information back and forth between RAM and the hard disk. This is called thrashing, and it can make your computer feel incredibly slow.