Basic difference between An Application Server and a web erver is that A Web server has no support for running EJBs. It Only provieds support for running web-applications (having JSPs, Servlet, plain Java classes etc.). If at all your application requires Support for Enterprise Java Beans then to host your EJBs you require an Application server. thats the most basic difference bn. and Appserver and a Web-Server.For more information try a lil bit of googling and browse thru java.sun.com.