A Web application is comprised of one or more related servlets, JavaServer Pages technology (JSP files), and Hyper Text Markup Language (HTML) files that you can manage as a unit.
The files in a Web application are related in that they work together to perform a business logic function. For example, one of the WebSphere Application Server samples is a Samples and applications Simple Greeting Web application. This application, comprised of a servlet and Web pages, greets new users when they access the application.
The Web application is a concept supported by the Java Servlet Specification.
Web applications are typically packaged as .war files.
Related reference
Default Application
Web applications: Resources for learning