JAD files are application descriptor files used as part of the
deployment of J2ME applciations (applciaitons for hand held wireless
devices).

A JAD file is a Java Application Descriptor, and is used by the various
JavaME technologies, mainly the MIDP. It's a descriptor that lets a handset
decide whether to download a related JAR file. The descriptor will contain
things like the size of the JAR file, the version of the application and
the amount of data storage that the application(s) in the JAR file will
require. Based on that information the handset can decide whether or not to
then download the JAR.