A CPU (Central Processing Unit) is the part of the computer that performs all of the complex calculations(The Brain if you like)it works in conjunction with many other parts but is usually the classed as the main part because the faster the processor the faster your PC will run programmes.
The processors power (speed) is measured in MHZ i.e a 550 processor will run at 550Mhz a 1Gig processor will run at 1000Mhz (unless its a Cyrix or Athlon XP as they use PR ratings) and is nearly twice as fast!