Aircraft Communications Addressing and Reporting System (ACARS) is a digital datalink system for transmission of short, relatively simple messages between aircraft and ground stations via radio or satellite. The protocol, which was designed by ARINC to replace their VHF voice service and deployed in 1978, uses telex formats. SITA later augmented their worldwide ground data network by adding radio stations to provide ACARS service. ACARS will over the next 20 years be superseded by the Aeronautical Telecommunications Network (ATN) protocol for Air Traffic Control communications and by the Internet Protocol for airline communications. 

The airlines, in an effort to reduce crew workload and improve data integrity, introduced the ACARS system in the late 1980s. (A few initial ACARS systems were introduced before the late 1980s, but ACARS did not start to get any widespread use by the major airlines until the latter part of the 1980s.) Although the term ACARS is often taken into context as the datalink avionics line-replaceable unit installed on the aircraft, the term actually refers to a complete air and ground system.

On the aircraft, the ACARS system was made up of an avionics computer called an ACARS Management Unit (MU) and a CDU (Control Display Unit). The MU was designed to send and receive digital messages from the ground using existing VHF radios. On the ground, the ACARS system was made up of a network of radio transceivers, which would receive (or transmit) the datalink messages, as well as route them to various airlines on the network. 

Examples of use:  

The system can be configured by an airline for whatever information they what to know. Such as, if an aircraft enters into a stall, engine (s) overheating, excessive airspeed (kind of like if the pilot was speeding), instrument errors, etc. The airline can as well set the time interval that information is sent back to the ground station.    

A captain who over heated an engine on start, decided not to tell anyone. Successful second start, and he took off. However, the ACARS sent out a report to ground crew of an engine overheat. As a result of failing to report the incident the pilot was demoted.  

Pilot and copilot on final approach got into an argument, resulting in stalling the aircraft. They managed to get the aircraft under control and land the plane safely. The ACARS sent out a report to ground crew of the stall. Both pilots were fired.