A WAR (Web Application Archive) file is a JAR (Java Archive) file that contains all the files, resources, and dependencies required to deploy a web application in a single package. A WAR file is a platform-independent format that can be deployed on any Java application server or servlet container.

Here are the contents that can be included in a WAR file:

1. Servlets and JSP files: These are the core components of a web application that handle HTTP requests and generate HTML responses.

2. Web pages: These are the HTML, CSS, JavaScript, and image files that make up the user interface of a web application.

3. Configuration files: These are the files that configure the web application, including web.xml and context.xml files.

4. Libraries and dependencies: These are the JAR files that contain third-party libraries and dependencies required by the web application.

When a WAR file is deployed to a Java application server or servlet container, the server extracts the contents of the file and creates a web application context. The web application context is a separate entity from other web applications running on the same server and contains all the resources required by the web application.

To create a WAR file, you can use a build tool such as Apache Maven or Gradle, or you can create it manually using a tool such as the jar command. Once the WAR file is created, it can be deployed to any Java application server or servlet container.