How a Cruise Control Actuator Works 

Here's how a cruise control actuator works on a car with a drive by cable throttle body.
Cruise control is used to maintain a constant speed without input from the driver's foot via the gas pedal. This can be a savior on long distance trips, reducing driver fatigue and increasing fuel economy.
On a drive by cable car, the actuator is situated between the gas pedal cable and the throttle body. It has two plates that move independently, with an electronic motor that can over ride its output to the throttle.
Inside the actuator is a small circuit board with two limit switches, a few diodes and capacitor. The limit switches tell the actuator the start and stop positions of the motor. Further inside, there are two motors, one solenoid type that acts as an electronic clutch, and the second motor actually turns the cable trolley via a spiral gear-train.
The electronic clutch engages the cruise control motor to the throttle body plate in the actuator. Its essentially an electromagnet that attracts a free spinning gear (attached to the drive motor) to lock up into the spindle when engaged.
The amount that the actuator rotates is controlled ultimately by the ECU, based on the driver's set speed and the vehicle's current speed. It is a closed loop control system that is constantly monitoring vehicle speed for any slight changes from external sources, such as a hill or a headwind, and adjusting the throttle position accordingly.
Replacing a cruise control actuator is fairly straight forward, with two cable connections, a 4 pin electronic connector and 4 Phillips screws attaching it to its mounting bracket.
On drive by cable cars, the throttle is already driven directly by an electric motor. The ECU controls the throttle via this motor for both normal throttle acceleration and cruise control.