Both do the same job i.e. they rotate the camshaft at half the revolutions of the crankshaft. However the important distinction is that cam chain has a long life expectancy and wear & stretch will become obvious with increased noise coming from the chain. Cam bekts being of a rubber compound have a finite life span - normally around 50,000 - 70,000 miles and must be replaced according to the manufacturers schedule. Failure to replace will result in a broken cam belt and catastrophic engine damage. Be warned !!