As the fuel in the gas tank gets warm some of it evaporates. These fuel vapors would increase the pressure in the fuel tank if not vented somehow. Instead of venting unburned gasoline to the atmosphere, our cars send these vapors to the "Charcoal Canister" The charcoal absorbs the fuel vapor, and stores it. Then under the correct conditions, the computer opens up a valve and flushes those vapors into the intake system.
This canister has no affect on power. Removing it can cause the car to smell of fuel when hot (either due to driving, or just hot weather).
hope that answered the question
jason