Until recently my understanding had been that there was only one computer on a car and that was the Engine Control Unit. However, it appears that this is not necessarily the case. Some cars may have more and this is an important concept to know when communicating with the car (The airbag system may be one and perhaps the ABS system another and of course there is the locking system etc).
These computers share what is known as a bus and is the connection by which they talk to each other. The communication standard does appear to have evolved over the years but the predominant one now in use would seem to be the CANBUS. The CAN standing for Controller Area Network. (It's quite a revelation to think my car includes a computer network)! This concept of a network of computers (more accurately called controllers because they are computers dedicated to certain tasks) becomes important when understanding the language used for managing the CANBUS.
