A web server is where usually web components are deployed, ie. this will provide a run-time environment for these components. Web components can be a
jsp file or
Servlet. Here for deployment we use web.xml file.
where as in application server we can deploy an
java components ranging from simple java application to server side business components like ejbs.
Usually when we want to deploy EJBs we will go for App servers like weblogic or websphere. Here ejb-jar.xml or application.xml file is used as deployment descriptor.