A traditional gasoline engine needs a mixture of air and fuel to create combustion inside the individual cylinder chambers (be they numbered 4, 6, 8, or 3 if you drive a DamnElantra®). If you simply let in a bunch of air and an Inland Empire Meth Lab amount of fuel, your car would first go Pah(!) then Kaboom. If you restrict the mixture of air and fuel too much, you will get no such Pah. This air-fuel mixture passes through the intake valve before being compressed and ignited, creating the car-go-now that we all seek. Post ignition, it’s shuffled out through the exhaust valve. Whence, four strokes: intake, compression, power and exhaust.
What determines when it is air-fuel feeding time for your hungry, hungry pistons and when it is time to let them expel their sweet, sweet exhaust gases? The Camshaft! Hop this here jumpsy-daisy to find out how.

First, we need to take a moment and explain a cam, singular. Essentially a circle with a bump on it, cams have been used by engineers for years to turn rotational motion into linear motion. Essentially, you have a circle with a bump on it with something the rests on said circle while it turns. When the bump comes along, whatever’s resting on the cam moves up with the bump. String a couple of cams on a stick and you’ve got yourself a camshaft. Link the rotation of the camshaft to the rotation of your engine and link the bumps to the valves of the engine, and you can see the basic principle at work. On to specifics…
The terminology’s a little odd, not unlike opposing sides of a political issue who both refer to themselves as “Pro” something. Nowadays, we’re basically talking about two main engine configurations: overhead valve (OHV) and overhead cam (OHC). (For you old-skool guys, we’ll get into flat heads another day). Jargon aside, an OHV engine has the camshaft in the bottom-end of the block, near the crankshaft, with pushrods traveling upwards to activate the valves. An OHC engine has the camshaft in the head, with the valves directly actuated by the camshaft and longish timing belt or chain stretching from the crankshaft up to the camshaft to drive it.
Dual overhead camshaft (DOHC) engines have one camshaft for the intake valves and one for the exhaust valves. DOHC setups make for more tunability when it comes to some of the more complex aspects of camshaft timing, as the open/close timing of the intake and exhaust valves can be set independent of each other.
Nowadays, the standard for a “modern” engine uses a DOHC configuration coupled with a “mutlivalve” configuration. As a minimum, you need one intake and one exhaust valve per cylinder. If you’re trying to flow more air/fuel in and more exhaust out more valves are better, so why not have two intake valves and two exhaust valves? Easy with a DOHC setup. Audi (among others) is known for running five valve-per-cylinder engines, e.g. the 40V V8 or the 20 V 4-banger with three intake valves and two exhaust.
The .gif above is a DOHC setup, while the one below is a SOHC setup.
This is what people are talking about when they compare a typically American “pushrod” V8 to a more modern OHC set up. In general, all of the features that allow greater flow help at higher engine speeds, but tend to hurt at lower RPMs. Hence, a DOHC Audi 4.2L v8 will make 400-something HP at 8000rpm while the OHV GM 6.0L LS2 V8 makes 400hp at 5500rpm and more torque across its whole rev-range than the DOHC unit(fainbois: the numbers are rough, for the purposes of illustration so back off). The point is: DOHC = more power from less displacement at higher RPM. OHV = lower HP/liter, but more low-end torque. (Again, gross generalizations for instructive purposes).

I mentioned above that the cam lobes are controlling the amount of air allowed to enter into this automotive chamber of secrets. They do this by lifting the intake valves for a certain amount of time per stroke, known as “duration”. The more air/fuel mix allowed into the combustion chamber, the bigger the PAH(!) at higher RPMs. This is why you often hear muscle car fans talking about “bigger cams”… they have increased the amount of valve lift and duration by swapping in an aftermarket piece. That lovely blub-blub-blub you hear from the ‘69 Camaro next to you at the stop light? That’s the product of a really hot cam setup that’s only happy above 4000rpm.
I know this explanation is not as cool as the Ghost-Riding Diff Explanation Team. If you are still confused, head on over to the HowStuffWorks link at the bottom. They do a much better job than I at explaining the cam, though I hope my rambling excuse to show off two cool .gifs helped you just a bit.
[Source: HowStuffWorks]
[Extra Source: Mad_Science – because my first shot at deciphering the mystery of the cam… wasa nata so gooda]