The short-block is your engine's largest and most basic sub-assembly, and is composed of the engine block, crankshaft, rods and pistons. The cylinder head (or heads, on a V-configured engine) tops off the short-block assembly to make the long-block, and the manifolds, fuel, exhaust and ancillary systems bolt to the long block. While all these components are important, none quite dictate the engine's performance potential the way cylinder heads do.
The cylinder head's primary job is to cap off the engine's cylinders and provide a place for the fuel to burn. This recess -- known as the combustion chamber -- can take many shapes and forms, all specifically engineered to provide the type of performance that particular manufacturer is looking for.
Intake and Exhaust Ports
The intake and exhaust ports are holes cast into the cylinder head that allow fresh air and fuel into the combustion chamber, and allow hot exhaust gases to escape. Intake ports are typically much larger in cross-section than exhaust ports because the rising piston and pressures in the chamber help to evacuate the exhaust gasses.
Something has to control the airflow into and out of the ports, and that job falls to the valvetrain. The head's "poppet" valves open down into the chamber when a rocker arm pushes them open via a long stem attached to the top. When the rocker arm releases pressure, a valvespring forces the valve back up and into the closed position. On overhead-camshaft engines, the valves may ride on a set of cam followers (small levers that connect to the cam) or on the camshaft lobes themselves.