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Valves let the air/fuel mixture into the engine and the exhaust out of the engine. The camshaft uses lobes (called cams) that push against the valves to open them as the camshaft rotates; springs on the valves return them to their closed position. The Timing Belt performs a critical job, because, if it breaks, the camshaft will stop spinning and the piston could hit an open valve.
Certain engines with overhead camshafts have timing belts that have a limited life span. Car makers use these belts instead of more durable chains because chains are noisier and cost more to manufacture. Your vehicle owner's manual will recommend at what mileage the timing belt must be replaced. These intervals range from every 60,000 miles to every 105,000 miles.
The job of the timing belt is to turn the camshaft(s) at exactly 1/2 the speed of the crankshaft while maintaining a precise alignment. This means that the crankshaft will make two revolutions for every revolution of the camshaft. Engines will have at least one camshaft, or as many as four camshafts in some of the V-type engines. The camshaft causes the intake and exhaust valves to open and close in time with the pistons which move up and down in the cylinders. The valves must open and close at exactly the right time in relationship to the piston movement in order for the engine to run properly.
There are two types of engines that use timing belts. They are described as: "Interference Engines" and "Non-interference Engines." On an interference engine, if the timing belt slips even one notch, the piston can crash into an open valve causing serious engine damage by bending valves and breaking pistons. Non-interference engines will usually not self destruct, but in either case if the belt fails, the engine will immediately shut down leaving you stranded. Timing belts can fail without warning and on some vehicles, are almost as hard to check as they are to change. In most cases, your only protection is to change the belt at the recommended intervals. Timing belt replacement is not a cheap job but it is far less costly than the alternative. Some technicians may recommend that you replace the water pump during a timing belt job even if there is nothing wrong with it. This is because 90% of the labor to change the water pump has already been done with the timing belt job and some technicians consider it good insurance to replace the pump at this time. Ask your technician what his experience is with the water pump on your model car and look at how long you plan to keep the car. This way, at least you will be making an informed roll of the dice.