A servo consists of a Motor (DC or AC), a potentiometer, a gear assembly, and a controlling circuit. First of all, we use gear assembly to reduce RPM and increase the torque of the motor. Say at the initial position of the servo motor shaft, the position of the potentiometer knob is such that there is no electrical signal generated at the output port of the potentiometer.
Now an electrical signal is given to another input terminal of the error detector amplifier. Now the difference between these two signals, one comes from the potentiometer and another comes from other sources, will be processed in a feedback mechanism and output will be provided in terms of an error signal. This error signal acts as the input for the motor and the motor starts rotating.