Camshaft

Camshafts play a critical role for the basic function of an engine. As the camshaft spins, the lobes (called cams) open and close the intake and exhaust valves in time with the motion of the piston.  [20]

The most common types of camshafts are the single overhead cam (SOHC), double overhead cam (DOHC) and pushrod.

The reason to use a DOHC rather a SOHC is to allow for more intake and exhaust valves. More valves means that intake and exhaust gases can flow more freely because there are more openings. 

The main difference between the pushrod and the SOHC or DOHC is that the camshaft on the pushrod engine is inside the engine block, rather than on its head. [19]