Cylinder heads are a large part which bolts directly to the engine block (at least, with a gasket between them) and often1 contains valves, rocker arms, and in an over-head cam set up, the camshaft. Further reading.
A Cylinder Head Being CNC Machined
This is a "billet" cylinder head being CNC machined for a high performance Duramax application. This YouTube video hosted by CentroidCNC.
- Side-valve engines have valves based in the block, to the side of the piston. These "flat head" designs have no valves in the cylinder head, which just serves as a cap for the top of the combustion chamber. This design fell out of favor in the late 1950s.