A SOHC engine has a single camshaft above each bank (this means 2 cams on a V engine, 1 on an inline) that in turn actuates the valves for each cylinder. A four-stroke motor cylinder has to have at least 1 intake valve for air a fuel to enter the cylinder, and 1 exhaust valve, to emit the fumes created by combustion. Some engines have 3 or 4 valves for intake and exhaust, but this requires more camshafts to turn more valves, hence double overhead cam, same idea, more cams. Camshafts are driven via a timing belt or chain from the crank shaft.
An overhead valve engine has the valves at the top of the combustion chamber, same as the OHC, however, the valves are actuated via pushrods that link directly to the crankshaft.
A V6, with DOHC will have 24 valves (4 valves per cylinder X 6 cylinders) where as a V8 with 4 valves per cylinder will have 32 valves.
Hope I could be of some help, If you'd like to do some research, the easiest way I have found, is just search wikipedia.org, it will have just about anything you are looking for, and generally with decent pics.
1999 4.0L 4dr 4x4
3" BL w/ torsion crank, 33x12.50 ProComp ATs on 15x10 ProComps