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Answer:
RAM is also called primary memory. It's primary because on the motherboard, it sits right next to the CPU (the brains of the computer).

When you open a program, the program instructions are fed from your hard disk into the RAM modules. These instructions are then queued up, like customers in a Macdonals restaurant, waiting to be processed by the CPU.

Video memory, or VRAM as it's also called, is memory that is allocated for rendering graphics and images (stuff you see on the screen, durr). Some computers don't actually have VRAM (VRAM modules are seen on Video cards), so they use part of RAM to compensate.

You might see some cheap budget computers that are advertised with “Shared Graphic Memory” or “Shared VRAM”. This means that the some of its primary RAM is being used as VRAM.

Virtual memory is a personal system technique which gives an application program the impression that it has contiguous/continous working memory, while in fact it may be physically fragmented and may even overflow from the RAM on to disk storage.

Let's suppose you only had 25 megabytes of free RAM available. If you opened a program and it requires 50 megabytes of RAM, and you only had 25 left, the remainder 25 megabytes is queued on the hard disk. Thus Virtual memory 'fools' the program into thinking there is more RAM than there actually is.

You can see a picture of it here http://upload.wikimedia.org/wikipedia/co…