In J2EE, application modules are packaged as EAR, JAR and WAR based on their functionality.
Each type of file (.jar, .war, .ear) is processed uniquely by application servers, servlet containers, EJB containers, etc
Jar file (file with a .jar extension) is the normal Java Application archive and intended to hold generic libraries of Java classes, resources, auxiliary files, etc
This can be added in classpath for compilation and to run java program. Generally in web applications we keep these files in lib directory of the application.
Ex: ojdbc14.jar – This contains all the classes to connect the oracle database
Servlet-api.jar – contains servlet related classes
Web modules which contains Servlet class files,JSP Files,supporting files, GIF and HTML files are packaged as JAR file with .war( web archive) extension
War files (files with a .war extension) are intended to contain complete Web applications. In this context, a Web application is defined as a single group of files, classes, resources, .jar files that can be packaged and accessed as one servlet context.
All above files(.jar and .war) are packaged as JAR file with .ear ( enterprise archive) extension and deployed into Application Server.
Ear files (files with a .ear extension) are intended to contain complete enterprise applications. In this context, an enterprise application is defined as a collection of .jar files, resources, classes, and multiple Web application.