A CPU is shot for "Central Processing Unit" and is also known as a processor. The job of a CPU is to process the majority of functions and instructions from the computer (sound and video cards can process specific graphic and video instructions). It is best the get the fastest and latest CPU processing structure because this means it can process the computer's instructions at a faster rate. The speed of a CPU is recorded in MHz. Basically when buying a PC it is best to find the fastest motherboard and CPU, these are the most important components.