Short for "Quick Access Recorder" on Airplanes
Mentioned in the book Airframe by Michael Crichton, it's very similiar to a black box(Flight Data recorder) on a plane. The QAR is not a mandatory device, while a Flight Recorder is.
The QAR monitors faults in all the systems in the plane, and records all the info (time, place, etc) as well as airspeed and the like. The QAR logs can be downloaded and viewed by mantainence workers on the ground.
In the event of a crash, the Flight Data Recorder is analyzed and the accident could be reconstructed. However, the Flight recorders are notoriously buggy and malfunction often. Sometimes the QAR logs are used in place of the Flight Recorder, although the data isn't as complete.
The QAR is an optional piece of equipment and only some carriers install them. Because they are not regulated by the FAA, they can be anywhere on the plane.
I'd recommend reading Airframe, it's a great book that explains a lot about the industry as well as provides for a gripping mystery.