Your cars timing belt is responsible for maintaining the precision that's crucial to your engine's functions. Essentially, it coordinates the rotation of the camshaft and crankshaft so the engine's valves and pistons move in sync. The expected lifespan of your timing belt is specific to your car and engine configuration, usually between 60,000 and 100,000 miles. (You can check your owner's manual or look online for your car's service schedule).
The belt is a synthetic rubber strap that contains fibre strands for strength. It has teeth to prevent slipping, which fit into the grooves on the end of the camshaft and crankshaft. It's a simple part for such an important function, and when it snaps, things get more complicated. Unlike many car parts that gradually lose function as they wear out, a timing belt simply fails. Whether the belt breaks or a couple of teeth strip, the end result is the same. One minute your car will be running perfectly, the next minute it won't. You're in trouble if your car has an "interference engine" in which the valves are in the path of the pistons. If the camshaft or crankshaft moves independently in an interference engine, there will at least one valve/piston collision. The fragile valves will bend, and you'll be faced with a costly repair.