An ECU, or Electronic Control Unit, is a small computerized device that controls one or more of the electrical systems or subsystems in a car or other motor vehicle. It is essentially a computer with internal pre-programmed and programmable computer chips that is responsible for controlling a specific function. ECUs are used to control a wide range of functions in modern vehicles, including engine and power steering control, power windows, seats, HVAC, door locks, keyless entry, airbags, and even basic active safety features such as automatic emergency braking.

ECUs receive inputs from different parts of the vehicle, depending on their function. For example, a door lock ECU would receive input when a passenger pushes the door lock/unlock button on a car door or on a wireless key fob. An airbag ECU would receive inputs from crash sensors and from sensors that detect when someone is sitting in a particular seat. The ECU would then communicate to actuators to perform an action based on the inputs.

The location of an ECU in a car can vary depending on the make and model of the car, but it is typically located in the engine bay or somewhere in the front of the cars interior, such as the glove box or under the dashboard.

In summary, an ECU is a computerized device that controls one or more of the electrical systems or subsystems in a car or other motor vehicle. It receives inputs from different parts of the vehicle and communicates with actuators to perform an action based on the inputs.