Timing Belts, Fan Belts, Serpentine Belts
The timing belt, the fan belt and/or serpentine belt, because it snakes through your engine compartment, are what allows all the components of the engine to function in harmony with each other. If any belt breaks, you are going to get a ride in a tow truck! This is a very preventable problem.
One of the symptoms of either a worn or loose timing belt is if you hear squealing when you rev the engine. If you are starting to hear that, its time to bring in the vehicle for a check.