Main memory, also called RAM, is the physical memory unit in the computer. Virtual memory also serves as computer memory, but is actually hard drive space acting as temporary storage for computer processes.
Computers use as much main memory as possible when storing data to be accessed by the processor. When the computer runs out of physical RAM, it uses virtual memory space set aside on the hard drive.
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Main memory speeds up a computer, while virtual memory tends to transfer data at speeds much slower than physical RAM. Thus, you should only use virtual memory when absolutely necessary.
RAM comes in several different types. Old computers may use SDRAM, while newer computers (as of 2010) use some form of DDR RAM--either DDR1, DDR2 or DDR3. There are no different "types" of virtual memory, since it's all drive space. But you can use just about any storage device for virtual memory, including a thumb drive.