Servlets and other files can belong to a Web application (servlet group). Indeed, every servlet in the administrative domain must belong to a Web application whose classpath specifies where to find the servlet class file.
A Web application is comprised of one or more related servlets, JavaServer Pages (JSP) files, and Web pages that can be managed as a unit. For example, you can start and stop the Web application in a single action.
The files in a Web application are related in the sense that they work together to perform a business logic function.
For example, one of the WebSphere Application Server samples is an "Expiring Page" Web application comprised of servlet, Web pages, and JSP files that work together to replace an "expired" Web page with a new one on an expiration date the user specifies.
The Web application is a concept supported by the Java Servlet specification. For IBM WebSphere Application Server purposes, a Web application needs to "wrapped" in a Web application in order to perform important administrative functions, such as securing it.