It can also be used to burn your Processor if you are Overenthusiastic
Overclocking is a technique to increase the stock clock speed of a Processor (allowing it to function much faster).
Using Google usually turns insane no. of expert tutorials on Overclocking (and Cooling,etc).
Yes, For motherboards that don't support increasing the stock FSB (like Intel original mobos), The Processor cannot be overclocked. But you can use some software, Its extremely risky.