The timing belt is the connection between the engine's crankshaft and camshaft. You cannot see the timing belt because it is encased behind the front engine cover. As the toothed timing belt rotates around the crankshaft and camshaft sprockets, it maintains the proper timing of these major engine components. On certain types of engines the timing belt may also drive the water and oil pump, or a balance shaft. Not every engine has a timing belt.
Some engines have timing gears or a timing chain.
The timing belt can fail just like the alternator or air conditioning drive belts. Some vehicles have clearance built into the engine components, so that when a malfunction of the timing belt occurs, it will not cause internal engine damage. This is called a non-interference engine. Vehicles that can have serious damage to internal engine parts when the timing belt malfunctions are called interference engines. Common problems caused by a broken or slipped timing belt on an interference engine are bent valves and piston damage.