Answer:
PCM = Powertrain Control Module.
It's a dedicated real-time computer system that controls just about everything to do with the operation of the vehicle. They are quite small, rugged and reliable. While a PCM CAN fail, the simple reality is, they seldom do.
The PCM controls the timing and duration of the fuel injectors. It controls the spark advance and the EGR, transmission shift points and generally anything else that can affect the performance, emissions and economy.
The PCM collects data regarding the operating status of the engine, including operating temperature, air temperature, air flow, throttle position, vehicle speed, amount of oxygen in the exhaust, wheel slippage and anything else that the engineers may have considered significant or just plain "nice".
The PCM for a 1999 S series Saturn is behind the dashboard, just to the right of the glove box.