ACARS (aircraft communications addressing and reporting system) is a data communications system which allows flight information to be transmitted between individual aircraft and the company host computer. These communications utilize various providers and communications mediums depending on aircraft equipment and current position.
ACARS is designed for domestic and international use. The system will automatically scan for VHF communications and will automatically utilize any available authorized air/ground system depending on aircraft position.
Typical usage involves receiving your PDC (ATC pre departure clearance), weather updates, ATIS in text format, automatically sending block out time, off time, on time and block in times to the company, text communications with your dispatcher or maintenance and position reports. It's alot like instant messaging with the company. Many ACARS equipped aircraft also have printers.
I hope that helps