APS, or accelerator pedal sensor, is used in electronic throttle systems(also known as drive by wire throttle systems), in other words, systems that don't use a cable between the pedal and the throttle body....in GM cars, this is known as TAC, or throttle actuator control....the APS sends a signal to the PCM and the TAC module, which electrically controls a motor on the throttle plate......so the throttle plate is electrically moved, rather than by a cable from the accelerator pedal....
I'm sure everyone remembers the massive recall that Toyota had on their gas pedals, which included GM's Pontiac Vibe, a joint venture with Toyota....