The engine control module, or ECM unit, differs in location depending upon the make and model of the car, but it typically resides in the engine compartment. Other locations for the unit include under the driver or front passenger seat or beneath the dash near the glove compartment.
The ECM acts as the car's brain, receiving signals from sensors throughout the vehicle and transmitting signals to make adjustments to various systems as conditions change. The ECM is also responsible for alerting the driver if adjusting systems does not bring things back into normal operating range. It does so by flipping on the "Check Engine" light and storing away codes to make diagnosing the problem easier.