Modern engines don’t have nearly as many belts as they used to. In most engines, there is only one belt called the serpentine. The serpentine distributes rotational power from the main crankshaft of the engine to the various pumps and systems, to do things like run a water pump, turn the air conditioning compressor and run the alternator. The serpentine is made of rubber and fiber cord and in time wears. Keeping tabs on how healthy the belt is will help you replace it before it fails.