A servo (servomechanism) is an electromagnetic device that converts electricity into precise controlled motion by use of negative feedback mechanisms.
Servos can be used to generate linear or circular motion, depending on their type. The makeup of a typical servo includes a DC motor, a gear train, a potentiometer, an integrated circuit (IC) and an output shaft. The desired servo position is input and comes in as a coded signal to the IC. The IC directs the motor to go, driving the motor’s energy through gears that set the speed and desired direction of movement until the signal from the potentiometer provides feedback that the desire position is reached and the IC stops the motor.
The potentiometer makes controlled motion possible by relaying the current position while allowing for correction from outside forces acting on control surfaces: Once the surface is moved the potentiometer provides the signal of position and the IC signals the necessary motor movement until the correct position is regained.
A combination of servos and multi-geared electric motors can be organized together to perform more complex tasks in various types of systems including robots, vehicles, manufacturing and wireless sensor and actuator network (WSANs).
See a video demonstration of a simple servo: