Technically, in terms of the aeronautics, there is no difference. Most commonly, a helicopter has a single rotor that provides mostly vertical force, and is powered by an internal combustion engine. Then a smaller rotor that applies horizontal force as a means to counteract the spinning force from the main rotor. Drones, as we see them today, have more rotors, and power them by electric motors. Instead of a rotor providing horizontal force, the drone's vertical rotors spin in opposite directions. Matched pairs of propeller systems provide a balanced spin force. Point is, a helicopter can have multiple rotors, and a drone can have a single rotor, they are, effectively, interchangeable. The real difference between a drone and a helicopter is that a helicopter is designed for manned operation, while a drone refers more to unmanned operations.