Can someone explain what the Under Drive Pully does
 

Would someone mind quickly explaining what the crank drive pully does? What are the performance gains to be had on getting a light weight one? What are the risks of installing them?

So there is a underdrive and crank... Whats the difference?

Same thing. Theres multiple versions you'll find:

Stock crank pulley
Lightweight crank pulley
Underdrive pulley

-Stock is exactly that.
-Lightweight is the same diameter as a stock pulley - just lighter, to free up a small fraction of power.
-Underdrive is a smaller diameter pulley (usually lighter, also) - this allows less parasitic energy to be used to spin your accessory belt (and, of course, all accessories THAT turns - alternator, AC compressor, etc). Obviously, those are not advised if you have a lot of power going to your sound system :). How much power you can get from that varies by the motor...on an LS1 it was worth 5ish WHP. This car, probably less.

thx dude

You asked about risks and there are some of those too. The risks aren't due to the pulley itself but the installation procedure. It requires putting a lot of force on the stock pulley to get it off. This can cause problems with the seal and oil leakage.