Short for Electronic Control Unit, the ECU is a name given to a device that controls one or more electrical systems in a vehicle. It operates much like the BIOS does in a computer. The ECU provides instructions for various electrical systems, instructing them on what to do and how to operate. Below are two pictures of what an ECU might look like, depending on the vehicle.
There are a number of different types of ECUs, including an Engine Control Module (ECM), Powertrain Control Module (PCM), Brake Control Module (BCM), General Electric Module (GEM) and others. Newer vehicles can have as many as 80 ECUs and due to their increasing complexity, the programming involved in developing the ECUs is becoming more challenging to maintain.