The cruise control system controls the speed of a car the same way as a person does — by adjusting the throttle position. But cruise control actuates the throttle valve by a cable connected to an actuator, instead of by pressing a pedal. The throttle valve controls the power and speed of the engine by limiting how much air the engine takes in.
Commonly, two cables are connected to a pivot that moves the throttle valve. One cable comes from the accelerator pedal, and one from the actuator. When the cruise control is engaged, the actuator moves the cable connected to the pivot, which adjusts the throttle; but it also pulls on the cable that is connected to the gas pedal — this is why the pedal moves up and down when the cruise control is engaged.
Many cars use actuators powered by engine vacuum to open and close the throttle. These systems use a small, electronically-controlled valve to regulate the vacuum in a diaphragm. This works in a similar way to the brake booster, which provides power to the brake system.
The brain of a cruise control system is a small computer that is normally found under the hood or behind the dashboard. It connects to the throttle control, as well as several sensors.