Drones are army planes that can fly for hours and probably weight hundreds if not thousands of pounds... Quadcopters are toys that can fly for minutes and weight only a few pounds...
"Drone" covers all UAVs/RPVs whether small enough to fit on your palm or large enough to need a full size air strip to launch.
All UAVs are drones and all drones are UAVs. Not all drones are quadcopters but all quadcopters are drones. Even in the strictest definition, what we commonly call "R/C Planes/Helicopters" actually qualify as "drones."
You might be interested to learn that the FAA has strict regulations on the registration and operation of helicopters, and the licensing of their pilots.
