The best way to put it is making things run faster than they were designed to ;P Making a 1.6GHz processor running at 1.8+
Usally you have to unlock the procesoor somehow*guides can be found online* then get into your bios or find the jumpers on your mobo(for old OLD stuff now ;P) and set them to faster speeds.
If you don't know how to get into the bios, I wouldn't even try OCing... It's quite dangerous and sometimes you have to get into your computer, say to reset it if something goes wrong...