Quote Originally Posted by SiReal
SOHC = single over head camshaft
DOHC = double " " "

Basically, DOHC engines make more power (I believe through the configuration of the double camshafts timing and that) engine can run more efficiently.
You hit the nail right on the head. It also makes it possible to double up on the valves for more intake and exhaust. It's just there for performance. Look at old, and some new, v-8's... they have the camshaft right above the crankshaft, not over the valves. They also only had one camshaft. Can't control the intake and exhaust for different timing with out affecting the other. Not saying you can't tune a SOHC just takes a bit more work.