ECU

Engine Control Unit

An ECU is a computer that runs the engine in all moderd day vehicles, Introduced in the early 1980s with the advent of electronic fuel injection systems, these early systems where very primitive they just monitored the revolution of the engine, air flow in to the engine and engine temperature. The ECU then used this information gathered from these sensors to inject the right amount of fuel at the right time. The advantages of fuel injection over a carburettured engine was the degree of control that a computer had over fuel delivery, 1000s of times more accurate than that of a carburetor.

An engine control unit (ECU), most commonly called the powertrain control module (PCM), is a type of electronic control unit that controls a series of actuators on an internal combustion engineto ensure the optimum running. It does this by reading values from a multitude of sensors within the engine bay, interpreting the data using multidimensional performance maps (called Look-up tables), and adjusting the engine actuators accordingly.

Before ECUs, air/fuel mixture, ignition timing, and idle speed were mechanically set and dynamically controlled by mechanical and pneumatic means. One of the earliest attempts to use such a unitized and automated device to manage multiple engine control functions simultaneously was the "Kommandogerät" created by BMW in 1939, for their 801 14-cylinder aviation radial engine

In the present day an ECU is a very powerful control system capable of gathering hundreds of channels of information to control every aspect of the engine, even its own efficiency. 
The worse place in the world that you could put a sophisticated piece of electronics is under the bonnet of a car due to massive vibration, constant heat variations and intense radio interference from the ignition system added to the fact that the manufacturers must produce on a huge scale, this causes so many problems and design flaws with modern day ECUs.