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A WAR file is a JAR file with a specific directory structure for web applications. It bundles servlets, JSP pages, static resources (HTML, CSS, JS), libraries, and a deployment descriptor (web.xml) into a single archive that Java application servers know how to deploy. Drop a .war into Tomcat's webapps directory, and the server extracts, configures, and serves the application automatically.
The format follows the Java Servlet specification: `WEB-INF/classes/` for compiled Java code, `WEB-INF/lib/` for library JARs, `WEB-INF/web.xml` for configuration, and everything else at the root for static web content. This standardised structure means the same WAR deploys identically on Tomcat, Jetty, JBoss/WildFly, WebSphere, and WebLogic.
Modern Java deployment has moved toward executable JARs (Spring Boot) and containers (Docker), but WAR deployment remains common in enterprise environments. Build tools (Maven, Gradle) create WAR files with a single command (`mvn package` with war packaging).