A Gyrocopter, Gyroplane, or Gyro for short, can be considered a cross between a helicopter and a fixed wing airplane. A Gyro uses rotor blades like a helicopter, but uses a propeller for power, as does a fixed wing airplane.
The rotor blades on top of a Gyro are mounted on a free spinning bearing and teetering system. These blades get their lifting power from the air moving up through them. As they move through the air they spin like a windmill. This spinning produces lift.
The forward motion of the Gyro provides the air moving up through the rotor, and a propeller provides the forward motion. As with a fixed wing airplane, in the absence of engine power, gravity must provide this forward motion.
To differentiate a helicopter from a gyro is simple. In a helicopter the rotor blades are powered. This power to the rotor blades creates an equal and opposite torque on the helicopter fuselage. A tail rotor is required to counteract this torque. A Gyro does not create this torque and therefore does not need a tail rotor