DOHC stands for Double Over-Head Cam. Two strokes don't have cams. Four-strokes have valves, and need cams to open them. Cams may be mounted low, and use pushrods and rockers, or high. An Overhead cam puts the cam above the combustion chamber. This is generally called Single Over-Head Cam. It still needs rockers but not pushrods. A double overhead cam usually opens the valves by bearing directly on them.
Most modern fourstroke motorcycles are DOHC.
http://en.wikipedia.org/wiki/DOHC#Double_overhead_camshaft

