Advantages- usually more power due to straighter intake runners(no pushrod ports to obstruct -shape/airflow) and higher rpm limit due to lighter valvetrain, easier to run multiple valves for more...
one cam enters the ring with his gigantic robin hood helmet on. the other enters the ring with his squidward hat on his head singing who wants to be a millionaire. during that time robin Williams...
It is an engine that has it's one and only cam shaft in the head above the valves. Instead of having it below in the block. A double overhead cam has two cams in each head.