Remember-the only stupid ? is the one not asked! I agree w/previous post. The head gasket is under the cylinder head and the valve or rocker cover gasket is on top of the head to seal the covers down.
The head gasket goes in between the engine block and the cylinder head to keep combustion chambers sealed off from one another. It also helps keep coolant from leaking into the cylinders from the passages inside the block and heads.
The valve cover gaskets keep the valve covers sealed on top of the heads to keep oil inside the top of the head instead of leaking all over the engine.
If you buy a "head gasket set" it will usually contain the valve cover gaskets,valve seals,and many miscellaneous gaskets.
You can purchase valve cover gaskets seperately. I personally like the "Victor" gaskets as they're made of better materials(leather of some sort, I believe).