A JAR file is a Java archive (JAR) file used by the Java Runtime Environment (JRE), a framework used for executing Java programs. The file may serve as a program library or as a standalone program that runs if the JRE is installed on the computer or mobile device.
JAR files can contain .CLASS files, application resources, and an optional manifest file (META-INF/MANIFEST.MF). They are compressed using .ZIP compression and may optionally be digitally signed using the jarsigner tool included with the Java SDK. JAR files may be created with the jar tool, and they can be accessed using Java's java.util.jar API.
JAR files are used for some mobile games supported by Java-enabled mobile devices. They are also used by Mozilla Firefox, Thunderbird, SeaMonkey, and Sunbird to store themes and add-ons.
NOTE: The Java Runtime Environment is also referred to as the Java Virtual Machine (JVM). Sun Microsystems, the original developers of the JRE/JVM, were acquired by Oracle.