Mechanical governors often work on a relatively simple centrifugal flyweight and cam system: A set of weights spinning on the shaft are attached via cams to the control unit (e.g., throttle or blade pitch controller). When the shaft spins faster, the throttle/controller adjusts to decrease the speed.
A FADEC uses a computer program and digital inputs from many sources to maintain the engine at the desired parameters. Signals are sent to the fuel controller many times per second to adjust fuel delivery as needed. Other adjustments may also be controlled by the FADEC, such as Inlet Guide Vanes, bleed valves, or prop/blade pitch (turboprops/helos).