Explain JAR Archives ?

HTTP 1.0 uses a separate connection for every request. When you're downloading several small files, the time required to set up and tear down the connections can be a significant fraction of the total amount of time needed to load a page. It would be better if you could load all the HTML documents, images, applets, and sounds a page required in one connection.

One way to do this without changing the HTTP protocol is to pack all those various files into a single archive file, perhaps a zip archive, and just download that.

We aren't quite there yet. Browsers do not yet understand archive files, other than applets do. You can pack all the images, sounds, and .class files an applet needs into one JAR archive and load which instead of the individual files. Applet classes do not have to be loaded directly. They can also be stored in JAR archives. To do this you use the ARCHIVES attribute of the APPLET tag
< applet code="HelloWorldApplet"
width="200" height="100"
archives="HelloWorld.jar">
< hr>
Hello World!
< hr>
< /applet>

In this example, the applet class is still HelloWorldApplet. Therefore, there is no HelloWorldApplet.class file to be downloaded. Instead the class is stored inside the archive file HelloWorld.jar.
Sun provides a tool for creating JAR archives with its JDK. For example,
$ jar cf HelloWorld.jar *.class

This puts all the .class files in the current directory in the file named "HelloWorld.jar". The syntax of the jar command is deliberately same to the Unix tar command.