What is over clocking, what are the pro's and con's?

Over-clocking is making a component (usually the CPU) operate at a faster speed than it was rated for.

The pros are simple; when done correctly, your system will be faster. Some chips can be overclocked by 30% or more, though most are limited to more modest increases (~10%).

The cons...

1) Power consumption will increase.
2) You will generate more heat; possibly enough to fry chips. Even if it doesn't melt right away, running warm *may* shorten the life of your components.
3) Cost. A properly over-clocked system will have a better-than-average cooling system. Maybe it'll be as simple as installing a bigger, better heat sink... but those are not free.
Your more ambitious OC's will require liquid cooling though, and that can run a couple of hundred dollars by itself.
4) Stability. There is always a little trial-and-error. Sometimes, when you are running too close to the limits of what the chips are capable of, the system will start to flake out. That is your cue to dial it back a notch.
5) Risk. Sometimes you won't know you've gone too far until you let the magic smoke out. There is *always* the possibility of frying something pricy, most often the CPU.

Overclocking is setting your computer's processor to run at higher speeds than it is meant to. If done properly and with adequate cooling you can obtain some real improvements in processing speed on your system. If done improperly and/or without adequate cooling you may wind up with a fried processor.
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Another con is that by overclocking your processor, your RAM will also be forced to run at a faster speed, adding a chance of random crashes because the memory will not be able to read and write data correctly at higher speeds.

Over clocking?? I have no idea. Pros and cons - pro is a good thing, con is a bad thing

I am just after info as to gain knowledge. I want to build a bad ass PC and need to know what to look for in the components. Do you have any advise on what I should put in the case.

Over-clocking is making a component (usually the CPU) operate at a faster speed than it was rated for.

The pros are simple; when done correctly, your system will be faster. Some chips can be overclocked by 30% or more, though most are limited to more modest increases (~10%).

The cons...

1) Power consumption will increase.
2) You will generate more heat; possibly enough to fry chips. Even if it doesn't melt right away, running warm *may* shorten the life of your components.
3) Cost. A properly over-clocked system will have a better-than-average cooling system. Maybe it'll be as simple as installing a bigger, better heat sink... but those are not free.
Your more ambitious OC's will require liquid cooling though, and that can run a couple of hundred dollars by itself.
4) Stability. There is always a little trial-and-error. Sometimes, when you are running too close to the limits of what the chips are capable of, the system will start to flake out. That is your cue to dial it back a notch.
5) Risk. Sometimes you won't know you've gone too far until you let the magic smoke out. There is *always* the possibility of frying something pricy, most often the CPU.