The serpentine belt travels around all of your auto's accessories, powering them as the crankshaft turns. It will turn your alternator, power steering pump, air conditioning, and so on. In older cars, there could be individual belts for each accessory. With a serpentine, one belt travels around to all of them.
Follow the link below to watch a video

http://edenautomotive.mynapatools.com/2013/10/24/serpentine-belt/