The term 'overclocking' explains the process of running your CPU at a clock and/or bus speed that the CPU has not been specified for - reasonably, that speed is usually higher. Why Overclocking?

The tempting idea behind overclocking is to increase system performance at very little cost. In lots of cases you only require to change some settings on your motherboard to make your system run faster. In further cases you only have to add a few components (typically for cooling) to achieve the performance increase.
Formerly, overclocking was usually nothing over increasing a CPU's clock speed to that of the next higher model, for example a Pentium 120 to a Pentium 133. Now, with new bus speeds available on various motherboards, you can change the clock and bus speed of a CPU to values that do not officially exist. This new technique of overclocking is squashy an even higher performance increase than the classic one. It still gives you the capability to increase the performance of the fastest model of a particular CPU production line (e.g. P200 to 250 MHz, PPro 200 to 233 MHz).