Source : Free On-Line Dictionary of Computing

Java archive
     
         (jar) A compressed {archive}
        file containing {Java} {class} files, filename extension:
        ".jar".  The {Java Development Kit} contains a tool called
        "jar" for creating .jar files, similar to the standard {Unix}
        {tar} command.  As well as archiving and compressing the Java
        class files, it also inserts a "manifest" file which can
        contain information about the class files, such as a {digital
        signature}.  Combining class files into a single archive file
        makes it possible to download them in a single {HTTP}
        transaction.  This, and the {compression}, speeds up execution
        of Java programs delivered via the {Internet}.
     
        (2001}-02-03)