A WAR file is a compressed package containing Java-based web components and applications that are run on a web server. It is formatted the same way as a .JAR file, but includes additional information that tells the application server which Java servlet class to run.

WAR files may contain server-side utility classes (such as database beans and shopping carts), static web resources (HTML, image, and sound files), and client-side classes (applets and utility classes). They can be run on a web server using a supporting program such as the Tomcat Web Application Manager within Apache. Since WAR files are saved in an archive format, the contents of a WAR file can be extracted using a file decompression utility like Stuffit Expander.