GENERIC AUTOMOTIVE PROTOCOLS
For example HTTP (Hyper Text Transfer Protocol), SSL (Secure Socket Layer), TCP/IP (Transmission Control Protocol Internet Protocol) are protocol that come to mind that are used for internet communication. Then there is the WI-FI protocol for cell phone or laptop to router communication, then there is Bluetooth for cell phone to cell phone communication or computer to printer communication to mention but a few.
Likewise the scan tool (someone at the door) sends a signal (knock on the door) to the engine control module or the steering control module, or the gateway modules. The relevant module (you) receives the signal and decodes it, then determines (the nature of business) that the scan tool requests access to its internal non-volatile memory. Because the request signal had the appropriate greeting, the relevant control module reciprocates and allows its contents to be read. Thereafter the scan tool transmits an exit signal and the control module releases or shuts down communication. That basically describes a successful yet very simplified digital information interchange between scan tool and control module. However, all this is done in naughts and ones, the bits and bytes of binary code and is a lot more intricate than described but the objective was just to give you a general idea of function of a protocol.
CAN-bus is used for the Instrument Cluster/Suspension Network
CAN-bus is also used for the Convenience Network
CAN-bus or LIN or MOST is used for the Infotainment Network
CAN-bus is also used for the Diagnostic Network