First port of call is this:-
http://forums.overclockers.co.uk/showthread.php?t=138583
It's a bit old, but it still holds true.
Overclocking is the craft of making your PC components run faster than the manufacturer intended, and is generally done to the CPU, RAM and graphics card. It generally relies on a good motherboard that supports overclocking through features such as addtional voltage options and the ability to run at higher FSBs (the primary method of overclocking the CPU and RAM).
They can all be overclocked becuase:-
1. Manufacturers sell higher spec parts at slower speeds to meet demand.
2. They are conservative with the set performance to ensure the failure rate is low, overclocking is finding out how much further you can push it, before it stops operating correctly then backing off a bit from that point.
3. Components are designed for a set lifespan (probably greater than 10years), they can often be made to go faster with more voltage/heat resulting in less life, but still in excess of its usefull life.
4. Electronics can run faster if they are cooled better.
Jokester