The cylinder head
sits atop the cylinders and consists of a platform containing part of the combustion chamber and the location of the valves. The top half of the cylinder head contains the camshaft in a OHC (Over-Head Camshaft) engine, or another mechanism (such as rocker arms) to transfer rotational mechanics from the crankshaft
to linear mechanics to operate the valves (pushrod engines perform this conversion at the crank and use a rod to push a rocker arm that acts on the valve). Internally the cylinder head has passages for air to travel to the inlet valves from the inlet manifold, and passages for exhaust gasses to travel from the exhaust valves to the exhaust manifold.
The number of cylinder heads in an engine is a function of the engine layout. A true vee engine will have two cylinder heads one at the end of each vee.
The cylinder head is key to the performance of the internal combustion engine, as the shape of the inlet passages and ports (and to a lesser extent the exhaust) determines a major portion of the volumetric efficiency of the engine.