Overclocking is the process of forcing a computer component to run at a higher clock rate than designed or designated by the manufacturer.
Overclocking is usually practiced by PC enthusiasts in order to increase the performance of their computers. Some hardware enthusiasts purchase low-end computer components which they then overclock, thereby attaining performance of a high-end system, while others will overclock high-end components, attaining levels of performance that surpass the performance of the newest generation of computer hardware.
Commonly overclocked components include: processors, video cards, motherboard chipsets, and RAM.
http://en.wikipedia.org/wiki/Overclocking