Physical Versus Virtual Memory?
Physical memory is what actually exists on your DIMMs, whereas virtual memory is an area of your hard disk that is set aside for the system page file (also referred to as the swap file, or virtual disk). As your physical memory fills, the system will cache data that is likely to be accessed again soon to the page file. That way, it can reacquire it faster than it would if it had to reach out to a website or reopen a file, for example.