Hi A/C train, Buzz here. HLF-MD11 has it about right. At UAL the pilots love ACARS, they get and send all sorts of information that relates to that flight. It's a data-link that uses a VHF radio freq. to send a "beep" of data. Most of our airplanes use VHF #3.
They'll get a print of the METARS weather where they're going, transmit the passenger load, send maint. gripes, we can send a "Hey-yu" to summon a particular flight if we think the flight crew should know something, flight crews can get the gate information (where passengers connect to the next flight), and our newer airplanes automatically read/ transmit engine performance numbers for evaluation- the flight engineer used to have to write it down and we'd have to type it in.
It's just another way of sending company data, with a touch screen.
It's a VHF radio data link, no voice.... just a beeping modem sound. It's handy.