The valve cover gasket is made of a flexible material, such as rubber, while the head gasket is a soft metal, usually layered to take the tremendous pressure it takes to hold the head on the cyinder block. A typical head gasket replacement willcost you up to $2000 on some cars since the engine has to be partially dismantled. The gasket itself may only cost $100 or so. A head gasket job also involves a new valve cover gasket.
The valve cover is under only slight pressure, so its main purpose is to keep the oil from leaking over the engine and trap some crankcase gasses and duct them to the engine intake where they can be burned off without contaminating the atmosphere.