What does an overhead cam do?

A conventional cam has more moving parts as it is placed within the engine block and uses lifters, pushrods, and rocker arms that all must work together with the cam and the valves. By moving the cam to above the valve train, called an overhead cam, there are now less moving parts, the engine becomes more efficient (can mean more power), can reach higher revolutions per minute (revs), and generally is more reliable.