What is a 'timing belt' and what happens if it breaks while the engine is running?

TIMING BELT (or timing chain) maintains the precise synchronisation between: CRANKSHAFT (controls piston movement) and CAMSHAFT (controls valve opening/closing). INTERFERENCE vs NON-INTERFERENCE ENGINE: In an interference engine, the piston and open valve occupy the same space at different moments — timing belt failure causes them to collide instantly, bending valves or breaking pistons = catastrophic engine failure; In a non-interference engine, there's clearance — the engine simply stops without internal collision. Most modern engines are interference designs. REPLACEMENT INTERVAL: Most timing belts require replacement at 60,000-105,000 miles (check owner's manual) — preventive replacement is far cheaper than engine rebuilding.