The war/ directory uses the WAR standard layout for bundling web applications. (WAR archive files are not yet supported by the SDK.) The Eclipse plugin uses this directory for running the development server, and for deploying the app to App Engine.
When Eclipse builds your project, it creates a directory named classes/ in war/WEB-INF/, and puts compiled class files here. Eclipse also copies non-source files found in src/ to war/WEB-INF/classes/, including META-INF/ and the log4j.properties and logging.properties files. The final contents of the war/ directory make up your application for testing and deployment.
For details about the new project that the plugin creates, see the Getting Started Guide.
Esfand S on 28 Sep 10"How do I use the plugin with a GWT project built with Maven? Although GWT projects typically use the Ant build system, it is also possible to use GWT and the Google Plugin for Eclipse with projects built with Maven. We recommend using Eclipse for Java EE when developing Maven projects, because it allows you to modify your source code and resources during a debugging session and have the changes automatically reflected in your running application. To enable this behavior, you'll need to convert your Maven project to an Eclipse Dynamic Web Project: 1. Open the New Dynamic Web Project wizard. Set the Project name and any applicable options, then click Next. 2. On the Java page, remove the default source folder (src) and add your Maven source folders (e.g. src/main/java, src/main/resources, and src/test/java). Click Next. 3. On the Web Module page, set the Content directory to src/main/webapp and click Finish. 4. Import your project's source code and resources into the newly-generated project, and set up your build path. 5. Finally, follow the steps in the GWT + Eclipse for Java EE FAQ to enable GWT for the project and create a Web Application launch configuration."