An electronic control unit (ECU) is an embedded system ( Hardware ) in automotive electronics that controls one or more of the electrical systems or subsystems in a car or other motor vehicle.
Modern vehicles have many ECUs some of those are as following:
- Engine Control module (ECM)
- Powertrain Control module (PCM)
- Transmission Control Module (TCM)
- Brake Control Module (BCM or EBCM)
- Central Control Module (CCM)
- Central Timing Module (CTM)
- General Electronic Module (GEM)
- Body Control Module (BCM)
- Suspension Control Module (SCM).
You can watch the very good Video from Vector which describes the basics of ECU.