Aircraft Crew Alert and Reporting System
Used to downlink information to ground stations via VHF/HF (such as aircraft information) and uplink information to the aircraft (such as flightplans for the FMC).
ACARS is also used for airlines on-time statistics. It has various sensors for aircraft info...which help define OOOI times.
OOOI stands for Out, Off, On, In
Out as in Out of the gate, Off as in takeoff, On as in on the ground (landing) and In as in In the gate.
Airlines use this for on-time stats.
You can listen to it via an airband scanner on really high MHz like 3,000. Or really low like 2 MHz. You need a specialized scanner for this though. However I don't know how to decode it. Hope this helps though.