Serpentine belt. just start up the car, open the hood and look down the front of the engine, between the engine and the radiator. There's a long 3/4" wide ribbed rubber belt, driven by the crankshaft pulley at the bottom of the engine, which drives the water pumt, alternator etc. also travels around an idler pulley/wheel and tensioner pulley/wheel.
If this belt breaks, the alternator will stop working immediately, along with the water pump and, if you drive long enough, with it broken, the engine will overheat and you will blow the HG.
Good luck with your fix.