The timing belt is usually made of synthetic rubber and is responsible for turning the camshaft at the right speed in relation to the crankshaft. It is also known as the cam belt, and ensures that valves in the engine cylinders open at the correct time.
When a timing belt is replaced, correct belt tension is critical. Too loose and the belt will whip, too tight and it will whine and put excess strain on the bearings of the cogs. In either case belt life will be drastically shortened.
It is absolutely essential to replace timing belts at intervals specified by the car manufacturer, as if the belt snaps whilst in motion it can result in serious and irrevocable damage being done to the engine.