Unused computing power
Much of time the average personal computer is doing nothing. Even when a person is using their computer, most of the time the CPU is not working at its full potential. When there is nothing for the CPU to do, it performs an instruction that is effectively "do nothing". A computer that is a quad-core running at 3 GHz is capable of performing 12 billion instructions a second. Often less than 5% of that is used, with only short bursts using half or more). Prime95 and other distributed computing programs use this otherwise wasted resource. Computers that have a modern graphics card also have a powerful GPU that is largely idle much of the time.
For many years owners of larger computer systems, try to keep them busy to amoritize the purchase price across many projects. David Slowinski used his mprime program to take advantange of these 'wasted cycles'.