A JAR (Java ARchive) file is a file that contains the class, image, and sound files for a Java application or applet gathered into a single file and possibly compressed. When a programmer gets a Java program development kit, a small program or utility called "jar" is included. The jar utility lets the programmer create, list, or extract the individual files from a JAR file. In an enterprise, a Java application can be started with a set of JAR files for use during execution. An off-the-shelf open source package can be delivered as a JAR file and run with XML data.
On the Web, a JAR file containing an applet may accompany a Web page. By putting the applet components in a single file and compressing that file, download time is saved. Ordinarily, a browser user will not need to "open" or view a JAR file directly. It is opened when the Web page is received and the applet is in some manner initiated.