overclocking is making your processor of a pci/agp card or your main cpu run faster. Your cpu's "clock" speed means its working power and speed, so overclocking your cpu means it is making it work faster.
The best way to overclock is by going to your BIOS settings. This is usually done by press a certain button at the startup info/splash screen (usually delete, F1, F2 etc). Refer to your motherboard and pc manual for exact instructions.
From there on it is simply a matter of exploring the menus and finding the correct options and changing the multiplier and/or bus speed. Overclocking is sort of complicated to explain, so google it for exact instructions in how.
There are several things you MUST keep in mind:
-When clocking your cpu up, only do it in single steps. Notch it up one, start your pc normally. If you can run without crashes or glitches, then try turning it up more.
-Cooling is VERY important when it comes to overclocking. Ensure your heatsink (fan over your pc) works, and that your case's fans work. Overclocking can heat your cpu up dramaticly, and getting it too hot can damage your cpu
Hope this helps a bit. I'm no expert on overclocking so its best to google/wait for a more experienced member to post!
If you can help it, DONT OVERCLOCK! experiment by all means but if you see your computer as something that NEEDS to be there all the time best thing is to:
A) buy a whole new system if your old one frequently seems to resemble a snail.
B) Back up all data on the computer that wants to be overclocked and then do as Cup of Squirrels take it in tiny steps.
I have screwwed a few machines through overclocking over the years through lack of knowledge and plain iggorance. For Example i can killed a 300mhz celeron before because it had no fan being a slot processor, it only had a heat sink :-|. And more recently a 1Ghz pentium 3 i overclocked to 1.1ghz and for some reason that is still unkown to me it wiped the hard drive :-| luckly all important stuff was back up. Be very cautious with overclocking as it will be your undoing!!