A Dual Overhead Cam system is usually for use with engines that have more than 2 valves per cylinder -- generally 4, but some 3 value-per-cylinder endgines utilize a DOHC system.

The name referes to the idea of having one cam shaft per X number of valves -- and more valves means more cam shafts, usually two vales per cylinder per cam. It gives a lot more power, but it also means there is more to go wrong, as well, because you have twice the stuff of a SOHC if it is a 'V' block, or 4 times if it is a straight block. And lets not forget that adjusting the timing on a single cam system is much easier than the timing on a Dual cam engine.