In its most general definition, a servomechanism, usually just shortened to servo, is a device used to effect mechanical control at a distance. For example, a servo can be used at a remote location to recreate the angular position of a control knob. The connection between the two is not mechanical, but electrical or wireless, for example.
The most common type of servo is that mentioned, which gives positional control. Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force, though other types that operate on hydraulic or magnetic principles are available. 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 is amplified and used to drive the system in the direction necessary to reduce or eliminate the error. A whole science of this type of system has been developed, known as control theory.
Servos are found in many applications. For example, the motor that positions the read/write head of disk drives and CD players is a servo. Servos are used to operate the throttle of engines that use a cruise control. Fly-by-wire systems in aircraft use servos to actuate the control surfaces that control the aircraft.
Another device commonly referred to as a servo is used in automobiles to amplify the steering or braking force applied by the driver. In this form this device is not a true servo, but rather a mechanical amplifier.