The notion of a web application was introduced with version 2.2 of the Servlet API. Web applications are a very important concept in that they allow a portable way to structure and bundle applications containing servlets,
jsp, and other resources (images, html pages, etc.). The "glue" that binds this application together is an XML descriptor named web.xml (found in the WEB-INF directory off of the application root directory). Within web.xml you can specify things like servlet names, servlet mappings, welcome files, error pages, security constraints, etc. Prior to version 2.2 each servlet container had their own way of defining things like servlet mappings, so this really hindered the ability to port applications from one vendor to the next; web applications have solved this problem (for the most part).
Karl Moss,<br />Author of <a href="http://www.amazon.com/exec/obidos/ASIN/007222262X/ref=ase_electricporkchop" target="_blank" rel="nofollow">Java Servlets Developer's Guide</a> <br /><a href="http://www.servletguru.com" target="_blank" rel="nofollow">ServletGuru.com</a> the Companion site.