Overclocking your pc is a risky business to say the least. What it involves is "cheating" one of the major components in your machine to run faster than it is supposedly designed to run at. For example, if you have a 2.8Ghz processor, you can force it to run at, say, 3Ghz. The same with the Motherboard, RAM, graphics card etc. Anything that has a set running speed can, THEORETICALLY be speeded up, or overclocked.
The good news is better speed, which is always good.
Now the bad news, there is inherent risk involved with this. The max speed of components is set for a reason. Overclocking a component will, most likely, result in the item running hotter and even a small amount of speeding up can be enough to "let the magic smoke out" of a MB or processor, which is why I don't recommend it normally and I wont post how to do it. My pc is overclocked because I am too cheap to buy a new one and I am running on a clapped out piece of junk, but I have paid the price for this in other ways (read new MB, new graphics card)