Most cars today have two engine belts, a serpentine belt and timing belt. The serpentine belt powers various engine accessories, such as the air conditioner, alternator (providing electricity to the battery). The timing belt synchronizes the crankshaft and camshaft.
Failure of either belt can have serious consequences for the operation of your car. Typically the serpentine belt will need replacement every 40,000 to 70,000 miles. The timing belt may last 100,000 miles. These are just guidelines. It is important to have the belts inspected for unusual wear whenever your car is in for maintenance.
Certain symptoms call for checking the belts immediately. Symptoms such as squeaking noises from the engine, overheating, A/C failing, a burning rubber smell, dashboard light or gauge warning your electrical system isn't charging properly.
Just for fun, here's an example of why serpentine belts are called serpentine. We borrowed this diagram from the internet. It is for a Ford. Every car is different. This just gives you an idea of how much of your auto's operation depends on this belt.