Beginners Guides: Overclocking the CPU, Motherboard and Memory - PCSTATS.com
You go into the computer BIOS and carefully increase certain values (FSB, voltages) that can speed up your hardware. Your Front Side Bus (FSB) directly affects processor speed. It is the main thing you have to increase. Along with that is the CPU voltage, i.e. how much juice is being fed to the CPU. In order for it to run faster, it needs more juice (depending on how Much faster).
You have to be careful with voltages. I suggest reading a few overclocking guides to get a basic understanding.