Radio control
On a car or boat, a servo is usually used for steering.  Model sailboats use a servo to adjust the sail as well as the rudder, or steering device.  On airplanes and helicopters, servos are used not only for steering but for up and down pitch.  If the model uses an engine to make it run, a servo is used to control the speed of the engine.  A model which operates with an electric motor usually has something called an electronic speed control or "ESC" to control its speed.