CAN messages in vehicles?

How do different sensors in vehicles communicate their messages to a CAN bus? Does each sensor have its own ECU (Electronic Control Unit) that assigns it an identifier and translates its output into CAN language? I'm having trouble figuring out how CAN really works in automobiles. Is there a central ECU that takes all of the information from the sensors into consideration and outputs it onto the network, or does the CAN bus take all the information straight from the sensors, package it up, and shoot it out, allowing whatever application needs the information to access it? Any information would be greatly appreciated!|||The sensors for the engine such as pressure, temperature, and speed sensors are connected to the ECU via analog I/O (input/outputs) pins. The sensor outputs a current or voltage that the ECU can read as a temperature, pressure, or speed. The ECU then translates this information and then sends it out on the CAN bus so that any other ECU on the CAN bus can read if it needs it. There are plans out there to eliminate the need for the Analog I/O and have a ECU for each sensor so it can send out its own CAN messages.