A UAV quadcopter (AKA quadrotor) is a multipropeller unmanned aircraft that is lifted and propelled by 4 rotating propellers. As such, they are considered rotorcraft, because their lift is generated by a set of vertically oriented propellers/rotors, as opposed to fixed-wing aerial vehicles (like an airplane).
Quadcopters usually feature 2 pairs of fixed pitched props. Two propellers rotate clockwise (CW) while the other two rotate counter-clockwise (CCW). A microcontroller varies the speed of each motor (and therefore it’s propeller) to achieve balance. By changing the speed of each rotor, the proper total thrust can be generated. The flight controller (microcontroller circuit board) often employs inputs such as tri-axis gyroscope, tri-axis accelerometer, magnetometer, barometric pressure sensor, etc. to supplement the radio controller receiver user inputs and achieve self-stability and self-balance.