A servo or servomotor is an electromechanical device that moves a mechanical linkage via remote control. In R/C use, they are either directly operated via the receiver, or in series with a failsafe, which is connected to the receiver. Servos are used whenever mechanical action of a model must be done via radio control. On a surface-operated model, servos are used for steering on all types and throttle control primarily for engine-powered models, although a servo is used in conjunction with a mechanical speed control on some models. Sailboat models use a special, high-torque servo designed specifically for use as a sail winch. Aircraft utilize servos for manipulation of the control surfaces, ground steering, landing gear retract systems and engine throttle as well. They are mostly standardized and compatible with all radio equipment, with the exception of some older Airtronics equipment, which use a reversed polarity from standard. ParkZone and HobbyZone use propietary connectors and wiring as well, but may be moving toward standardization.
Regardless of manufacturer, servos all basically operate in the same manner. The electronic signal sent via the receiver is used to power the servo's motor in either possible direction and rotate its gears and output shaft. In a proportional servo (which constitute the vast majority of servos), the farther the transmitter control is moved, the faster the servo's drive motor begins to spin. A feedback potentiometer operated via the gears turns the motor off when the servo reaches its desired position. Operating the transmitter controls from that point once again turns on the motor and the process begins again.
Digital servos have quicker response times, but if used with an analog receiver, they will not have this benefit. They also take a few seconds after powering up to calibrate themselves. In theory, it's possible to get in a crash as the result of loss of calibration, resulting in loss of control for up to three seconds.
Coreless servos have a type of stepper motor instead of a geared system to move the output shaft, and they are always digitally controlled as well. These make lots of torque and speed and are fairly expensive. Many modelers agree that the sound of a coreless servo is a rather pleasant one.