The Engine Control Unit (ECU) is a central electronic command that registers sensor/gauge inputs across the engine and controls a series of actuators which optimise performance. Essentially this is a vast series of mapping tables of inputs and outputs. The sensor readings dictate the inputs, which the ECU then identifies the desired (or mapped) output and delivers that to the actuator.
An ECU is primarily responsible for controlling the fuel injection and ignition sequence of the internal combustion engine. It can also control idle speed, air-fuel ratio, anti-lock brakes and electronic fuel injection.
As analogue evolves to digital, the principle of multiple smaller independent ECUs became the norm. Automatic transmission have a dedicated ECU providing precision shifts. The suspension units may have an ECU controlling ride smoothness, while remaining suitable for the prevailing road surface. An ECU may control the internal electronics such as air conditioning, electric seats/windows, door locks.