FSB

From TheBestCaseScenario

The Front Side Bus (FSB) is usually one of the main factors in overclocking, increasing the speed of the FSB allows you to increase the Core clock speed to get more performance from a lower priced processor.

The FSB should be increased slowly to make sure you do not damage the processor itself. The FSB is multiplied with the Multiplier to result in the Core clock speed

Example: A FSB of 333MHz x a multiplier of 9x = 2997MHz or pretty much 3GHz

The FSB speed increases the speed at which the CPU 'talks' to the Northbridge, increasing the speed obviously increases the performance.