Over-Head Cam (often abbreviated as OHC) and Dual Over-Head Cam (abbreviated as DOHC) engines have their camshafts located above the head so they don't have to rely on pushrods coming from a cam located in the block.
Single Over-Head Cams
This design uses one cam per head (meaning a V6 or V8 engine with this design would have two total cams, while an inline-4 or inline-6 would only have one) so both the intake and exhaust valves are controlled by the same cam.
Dual Over-Head Cams
The DOHC design uses two cams per head (meaning a V6 or V8 engine with this design would have a total of four cams, while an inline-4 or inline-6 would have just two) so the intake and exhaust valves are each controlled by a dedicated camshaft.