Valve covers keep the valve chamber sealed while sometimes serving as an aesthetic element for the engine. There are as many valve covers as there are rows of pistons, at least on most applications.
Valve covers tend to have a valve cover gasket seal to keep the oil inside. These gaskets fill the gap between the valve cover and the cylinder head to prevent oil leaks.
Valve Cover Design
Some valve covers have the company name displayed on it in some way (etching, pressed, etc.), and in the case of hand-built engines, the main builder will usually have their name on it as well. They are often a way to easily identify what type of engine it is, or even the variation of a given set of engines.
Sometimes valve covers are painted, either from the factory or for a modification. Painted valve covers improve the look of the engine bay.