The jar file format is common for distributing programs and libraries that are written in Java. Jar stands for Java Archive. Your browser may download Java applets in a .jar file or you may get a .jar file that contains a Java application that needs to be run. The file format is also by Mozilla and Firefox as the format for XUL applications, plugins, and skins.
The jar file format is a container and compression file format. Each jar file compresses and contains other files. Jar files are very easy to open. There are many programs that will act as an opener for jar files. In fact, any program that can open zip files can open jar files. The jar format is identical to the zip file format.