The difference are two letter......The S and D.

Peace!!! By the way, Ill let someone else do the big answer, here is the short one. SOHC develops a little less power than its DOHC brother, but it develops more of its power earlier and is therefore more flexible and generally more economical. The DOHCs rev much harder and higher, and at mid to top revs develop much more power for the same displacement engine, making them ideal for racing or performance vehicles. And if the DOHC is a V-engine, like a V6 or V8, it could be called a Quad-cam.