The timing or cam belt connects the camshaft & crankshaft to control the timing of the Engine Valve opening and closing. For example, the valves in a four-stroke engine open and close once every other turn of the crankshaft.
The timing belt may also be integrated to drive the water pump and/or oil pump.
Lube Oil, Hydraulic Fluids & Fuel Oil can all leak onto the cambelt - timing belt and be absorbed into the rubber-polymer compound subtly altering its dimensions and physical properties. Any dimensional change and the belt will no longer perfectly mesh with the pulley teeth of the crankshaft and camshaft. This increases the stresses on the timing belt making premature failure more likely.
If timing belt failure occurs then pistons and valves no longer sequence correctly. If failure occurs on an interference engine then pistons and valves will collide. Impact causes piston & conrod damage, valve damage, cylinder head damage,camshaft damage and so on...
Recommended timing belt replacement intervals can be as low as 50,000 Miles and as High as about 100,000 miles. Typically, timing belts are changed every 60,000 to 80,000 miles.