What is the difference between a timing belt and a timing chain?

Both keep the crankshaft and camshaft synchronised so valves open and close in time with the pistons. The difference is material and, critically, maintenance.

A timing belt is reinforced rubber. It is quieter, cheaper to manufacture and lighter, and it wears. Manufacturers specify a replacement interval, commonly expressed in both distance and years, and that interval is not optional guidance. A belt that fails does so without warning.

A timing chain is metal and runs in engine oil. It is designed to last the life of the engine, so there is no scheduled replacement. It is not maintenance-free in practice: chains stretch over time, and the tensioners and guides that manage them wear, particularly on engines with extended oil change intervals or a history of neglected servicing. A rattle on cold start is the classic early symptom.

The reason belt replacement is treated so seriously is engine design. In an interference engine, the valves and pistons occupy the same space at different moments. If the timing is lost, they collide, and the result is typically bent valves and often a damaged cylinder head, which frequently costs more than the vehicle is worth. Most modern engines are interference designs.

In a non-interference engine the same failure stops the car without destroying it, but these have become rare because interference designs are more efficient.

Two practical points. The water pump is often driven by the belt and shares the same labour, so replacing both together is standard and much cheaper than doing them separately. And when buying a used car with a belt, documented evidence that it was changed on schedule is worth real money, because assuming otherwise means budgeting for the job immediately.