A servomechanism, or servo is an automatic device that uses error-sensing feedback to correct the performance of a mechanism.
The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback that controls position -- the operator does this by observation. By contrast the car's cruise control uses closed loop feedback, which classifies it as a servomechanism.
A servomechanism is unique among control systems in that it controls a parameter by commanding the time-based derivative of that parameter. For example, a servomechanism controlling position must be capable of changing the velocity of the system because the time-based derivative (rate change) of position is velocity. A hydraulic actuator controlled by a spool valve and a position sensor is a good example because the velocity of the actuator is proportional to the error signal of the position sensor.
A servomechanism may or may not use a servomotor. For example, a household furnace controlled by a thermostat is a servomechanism, yet there is no motor being controlled directly by the servomechanism.
excerpted from wikipedia