In an internal combustion engine, the cylinder head sits atop the cylinders and consists of a platform containing most part of the combustion chamber and the location of the valves and spark plugs. The top half of the cylinder head contains the camshaft in a overhead cam engine, or another mechanism (such as rocker arms and pushrods) to transfer rotational mechanics from the crankshaft to linear mechanics to operate the valves (pushrod engines perform this conversion at the camshaft lower in the engine and use a rod to push a rocker arm that acts on the valve). Internally the cylinder head has passages for the fuel/air mixture to travel to the inlet valves from the intake manifold, for exhaust gases to travel from the exhaust valves to the exhaust manifold, and for antifreeze (coolant) to cool the head and engine.
The number of cylinder heads in an engine is a function of the engine configuration. A V engine usually has two cylinder heads, one at each end of the V. A straight engine has only one cylinder head.
The cylinder head is key to the performance of the internal combustion engine, as the shape of the combustion chamber, inlet passages and ports (and to a lesser extent the exhaust) determines a major portion of the volumetric efficiency and compression ratio of the engine.
Close up of a BMW cylinder head