How an operating system’s use of virtual memory enables the operating system to appear to support the use of more memory than is physically installed in a computer
I'm not sure how to explain this in detail, but im thinking because virtual memory is based on paging, thus a single process can demand more memory than the amount of physical memory stored. Therefore it "appears" to use more memory than the amount of physical memory.
But Im not sure if that explains it :(
Basically, as the name states, the virtual memory doesn't "exists" or is not directly related to physical memory. The virtual memory of a process is stored on the disk, containing all the information concerning the real process. See virtual memory for some more insights. When a process is scheduled in by the processor, some parts of its memory is brought back into the main memory through memory swapping. The pages that are needed at that moment by the process are in the main memory. And the pages that are not that much used (by any process) are just swapped-out because the main memory can't contain everything at the same time. Hope I helped :)