When you peek under the hood of nearly every modern car, you won't be staring at the engine, but a protective plastic or metal covering. This guard is in place to make the engine compartment more aesthetically pleasing, as well as protect the engine parts. There are three different types of covers that are used depending on the type of engine your car has. Most vehicles have an overhead-cam engine, and a cam cover is used to provide some protection from the elements. There are also rocker covers used for HEMI engines and V8s.
The cam shaft is a spinning rod with lobes (cams) that extend down to open and close the valves in time with pistons. This facilitates fluid intake of fuel and expulsion of exhaust gases. There are two types of this engine style, single overhead cam (SOHC) and double overhead cam (DOHC), and you'll either find one or two rods atop your engine when you have a cam setup. If you've just bought your car and want to ensure everything is working properly or you need to get to the engine to make a repair, you'll need to remove the cover to access the cam shaft and the rest of the engine.
When you do this, it can be a good idea to take some time to clean the cam cover. It is not an essential task, but it can make your engine bay look nicer. The cover is bolted on and likely has a few gaskets in between the cover and the engine. Be sure to check with the owner's manual to determine exactly how the cover is affixed to the engine for your particular make and model. This will also specify the proper torque needed to replace the bolts when you're done.
Disconnect the car battery and move any hoses or wires out of the way so you will be able to safely and easily remove the cover once you've unbolted it. Then it's time to remove the bolts. Make sure to keep them together in a dish or a bag so you don't accidentally lose one. Once all the bolts are taken out, you should be able to lift the cover right off the engine. You'll probably want to wear gloves for this, as the underside will likely be a bit grimey.
If the cover is stubborn, make sure you haven't missed any bolts. It may also be stuck if the car is older or you've never removed the cover before and it is glued in place. You can use a rubber mallet to loosen the cover by gently hammering on a corner of the piece. You should forego a metal tool for this, as you could dent or break the cover. Once you've got a bit of lift, use a flathead screwdriver to reach under the lip and help pry the cover from the gasket. Patience is key to avoid damaging the gasket or the cover. Work slowly and apply light pressure.
Once the cover is removed, you'll be able to see your engine and inspect it for any signs of damage or make repairs. When you're done and ready to replace the cover, you might want to clean it first, as you probably won't have another opportunity to do so for a while. You'll need a cleaner designed to fight grease, such as paint thinner, and a brush to really get in there and remove the grease. Be sure to wear protective gloves and eyewear when working with degreasers. When you're done, you'll need to replace the gasket before you put the cover back on. Then, simply put the cover in place and refasten the bolts with the specified torque.