What is a WAR file? Learn about WAR files here
A WAR file is a actually a JAR file with specific content. WAR files are a Web Application aRchives.
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WAR file details
A WAR file has a special folder structure and contains special files in addition to JSP pages, Java servlets, Java classes, HTML pages etc. which combined forms a Web Application.
Opening WAR files
If you are trying to open or create WAR files on a Windows PC, then you will need a tool like BitZipper. After installing BitZipper, you can open a WAR file simply by double-clicking it or by right-clicking on it in Windows Explorer, or by starting BitZipper and opening the WAR file the same way you open a document in your word processor.
Notice that this is for viewing what is inside a WAR file, not for starting the Java application in the WAR file.
If you are looking for technical details and the complete history of the .WAR file format, please refer to this Wikipedia article and the WAR specifications at Sun Microsystems.