Java Archive (JAR) files are compressed files into which you can store many files. If you place the many classes that your application, or applet needin a JAR file, you reduce the size of your distributabels.
To create a JAR file, go to the directory where your files are located and type:

 jar -cf ajarfile.jar *.*

and every file and directory in the current directory will be in a filecalled ajarfile.jar.
If you like to see what the JAR tility adds to the jar file, type:

 jar -cvf ajarfile.jar *.*

The “v” option sends the JAR utility to a verbose mode, printing outinformation about its activities.
If you want to extract the content of an existing JAR file, type:

 jar -xf ajarfile.jar

Again, if you like to see files (or directoies) that the JAR utilityextracts from the JAR file, use the “v” option, as in:

 jar -xvf ajarfile.jar

If you like see a table of content of a JAR file, type:

 jar -tf ajarfile.jar

Again, you can use the “v” option to see some information about the files in the JAR file, as in:

 jar -tvf ajarfile.jar

There are other options with the JAR utility, to see them just type:

 jar

If you have a UNIX background, you might have realized the definitesimilariries beteen JAR options and the options of UNIX tar utility.