This is a closeup of one of the servos used in my robot Maxwell, and these are the servos that work with the two servo control boards available for sale in my Products section.

A servo like the one at left is most commonly used in radio control models. For instance, a radio control car would have a servo controlling the steering. Radio control planes use these same servos, used to move the flaps and so on to control the plane's flight.

What the servo has inside is a small electric motor, with some gears attached so the output axle turns much more slowly than the electric motor but with more torque. Also, there is a sensor on the output axle to sense it's position. There is circuitry inside the servo that monitors the position and turns the motor to keep the output shaft in position.

This is the same concept used in industrial servos, and the name servo refers to that system of motor with sensor and control electronics.

Want to learn more? I have some details of Maxwell's construction here.