electric motors controlled by radio receiver. Normally two, one to control rudder via pushrod and one to control sails via sheets.

The radio components that do the physical work in an airplane, by moving rods that are connected to various parts of the plane.

Little black boxes slaved to the Receiver which move in accordance to the Transmitter's commands. You need one servo for each Channel - R/C cars only need 2-channels.

Servos apply certain mechanical functions. For instance, in an automatic transmission, if first gear is actuated by the application of a band, the servo is what applies that band. The servo is nothing more than a mechanism (either hydraulically, vacuum, or mechanically operated) that is connected to a mechanical linkage, that when actuated by it's governing force, moves whatever it must to perform a function, when the need for a certain application arises, the servo makes that application.