Java Application Descriptor file
JAD files describe the contents of a MIDlet, a type of Java application commonly used for games and applications for Java ME (Java Micro Edition formerly known as J2ME) for mobile platforms and embedded systems. JAD files are used to identify retrieve and install MIDlets.
Learn more about JAD files:
Michael Juntao Yuen discusses the challenges of Java ME development.
Wikipedia has an entry about Java ME.
Search for answers to JAD-related questions at IT Knowledge Exchange.