A crankshaft refers to the main rotating shaft in an engine. Thus a crankshaft pulley is a grooved wheel which is attached to the front end of the crankshaft which is connected by accessory belts to the fan, alternator, power steering pump, water pump, air conditioning compressor, and other devices so that the rotating crankshaft can drive these other parts as well. It has timing marks located on it, and these are used for checking and adjusting timing.