A quadrotor, also called a quadrotor helicopter, quadrocopter or quadcopter, is a multicopter that is lifted and propelled by four rotors. Quadrotors are classified as rotorcraft, as opposed to fixed-wing aircraft, because their lift is generated by a set of revolving narrow-chord airfoils. Unlike most helicopters, quadrotors generally use symmetrically pitched blades; these can be adjusted as a group, a property known as 'collective', but not individually based upon the blade's position in the rotor disc, which is called 'cyclic' (see helicopter). Control of vehicle motion is achieved by altering the pitch and/or rotation rate of one or more rotor discs, thereby changing its torque load and thrust/lift characteristics
Introduction (from Wikipedia)
A helicopter (informally called "chopper" or "helo") is a type of rotorcraft in which lift and thrust are supplied by engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards.
A quadrotor, also called a quadrotor helicopter, quadrocopter or quadcopter, is a multicopter that is lifted and propelled by four rotors.