It's difficult to imagine the extent of the damage that a cracked valve cover or missing valve cover bolts can cause your engine. Since a lot of oil is running through the top of your cylinder head, this cover makes sure that none of it leaks out into the engine bay. If it does, then it will coat all the components near the leak. They will, as a result, attract dirt and dust that is virtually impossible to get rid of. Not only does this make your engine bay cosmetically unappealing, but it could have a negative effect on the performance of some of your moving parts like drive belts and accessory pulleys. It will also get all over your ignition wires, exhaust manifold, coolant hoses, etc. So when you notice a little bit of oil coming from you valve cover or its bolts, get them fixed ASAP.
Depending on your engine's configuration, it may not have these. That, of course, is only the case for engines that did not have rockers and valves in the cylinder head such as side-valve or flat-head engines. But in the 1940s, the overhead valve engine revolutionized design and efficiency. This employed a single camshaft in the center of the block to push rods up against rocker arms which, in turn, pushed the valves down into the combustion chamber. Oil pathways were included in the cylinder head, therefor requiring a cover to be bolted to the top. This protects the rockers, valve, springs, and the rest of the engine bay. Your valve cover will be bolted to the top of the head, and have a gasket sealing it along with grommets to seal each bolt. Modern overhead cam engines will have more complicated valve covers that protect camshafts, cam seals, and maybe some timing components.
Installing a valve cover is super easy. Often times, they are only held in place with 6 to 10 bolts and grommets. You may have to remove some PCV hoses or valves to get it off the engine, but those are fairly easily accessible. But just because valve covers are easy to remove and replace, doesn't mean they should be ignored. Leaking oil is bad no matter where it comes from, so don't just think you can tighten the bolts or get new grommets and expect the problem to go away.
At Car Parts Discount, we know the need for quality engine parts. That means that when you see replacement valve covers or the bolts and grommets that fasten them to the cylinder head, you are seeing high quality parts that will fit the first time and perform the way your original equipment did (before it broke).