The Central Processing Unit (CPU) or (micro) processor is the part of a computer where most calculations take place. In terms of computing power, the processor is the most important element of a computer system, so the performance of a computer is highly dependant on the performance of the processor. For quite some time the performance of a processor has been determined by the number of MHz the clock speed was, but nowadays clock speed doesn't have so much to do with the speed of the processor anymore. One of the best ways to compare different kinds of processors, is to look up benchmarks on the internet. Please be careful to buy a processor that is compatible with your motherboard, i.e. fits in the right socket.