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.