What's a .jar file?
A JAR file or Java ARchive is a way of bundling multiple files together as a single file. Like a ZIP file, they support compression and retain the directory structure of the files. They can be used for delivering things like JavaBeans and Applets, and usually include support files like image and sound files, in addition to the actual Java classes. In addition, they can be digitally signed so a user can grant privileges to allow the software to perform operations not normally permitted from untrusted software.
To create a JAR file, use the jar tool that comes with the JDK.