From Forum Nokia Wiki

A JAR File (Java ARchive) is a ZIP file used to distribute a set of Java classes. It is used to store compiled Java classes and associated metadata that can constitute a program.

In Java ME it's the format in wich an application or game is distributed. It can be generated by a SDK, by a command-line tool, or by using an IDE, like NetBeans or Eclipse.

A Java ME JAR file can include:

• Compiled and preverified classes (Midlet and other).
• Icons, graphics and images.
• Data information (records, internationalization labels, game levels, etc.)
• Manifest file

A JAR file usually is distributed with a JAD file, describing what the JAR has.