A timing belt is a loop of a flexible yet strong material, usually a polymer of some sort, that is used in an internal combustion engine to transfer motive power from the crankshaft to the camshaft(s). For an excellent description of what a camshaft does, see Internal Combustion Engine : A Discussion of Valve Actuation Schemes by Starrynight. If the timing belt or chain in an engine breaks, you will end up with a severely damaged engine, since the valves will be left protruding into the cylinders while the pistons continue to move, resulting in shrapnel.