Main memory is the R.A.M. (Random Access Memory) of a computer system. R.A.M. is your computers short term memory. When you run a computer program the program itself is stored in the R.A.M. while you are working with the program. This kind of memory allows you to modify a program, file, document etc on your computer. When you perform a save the information is moved from the computers R.A.M. and stores it on the hard drive for long term storage.