A common type of servo provides position control. Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force.
Other types of servos use hydraulics, pneumatics, or magnetic principles.
Usually, servos operate on the principle of negative feedback, where the control input is compared to the actual position of the mechanical system as measured by some sort of transducer at the output. Any difference between the actual and wanted values (an "error signal") is amplified and used to drive the system in the direction necessary to reduce or eliminate the error. This procedure is one widely used application of control theory.
Servomechanisms were first used in military fire-control and marine navigation equipment.
Today servomechanisms are used in automatic machine tools, satellite-tracking antennas, remote control airplanes, automatic navigation systems on boats and planes, and antiaircraft-gun control systems.
Other examples are fly-by-wire systems in aircraft which use servos to actuate the aircraft's control surfaces, and radio-controlled models which use RC servos for the same purpose.
Many autofocus cameras also use a servomechanism to accurately move the lens, and thus adjust the focus.
A modern hard disk drive has a magnetic servo system with sub-micrometre positioning accuracy.
Typical servos give a rotary (angular) output. Linear types are common as well, using a screw thread or a linear motor to give linear motion.
Another device commonly referred to as a servo is used in automobiles to amplify the steering or braking force applied by the driver. However, these devices are not true servos, but rather mechanical amplifiers.
In industrial machines, servos are used to perform complex motion.