JAR : Java Archive
JAR file are basically archive file that are created using the Java Archive tool which comes with JDK. It is similar to "ZIP" files which are generated using WinZip utilities.
The main purpose is to combine the files that are used by the applet (like image files, class files) into a single compressed file for efficient loading of the applet by the Browser.
Now In a single HTTP connection the web Browser is able to download the entire classes that the Java applet could have requested when a particular class is required in the execution (interpretation) process. This reduces the communication between the client and server.