HttpServer = WebServer THe sole job is request / response typically of HTML pages. Example, Personal Web Server that comes inbuilt with Win 98 or IIS. (But IIS can process ASP pages, so its an App Server too).
Nowdays J2EE AppServers come bindled with a WebServer. Examples of full blown J2EE app servers are Weblogic, Websphere, JBOSS etc.
Tomcat is a web-container only and NOT an enterprise bean container and hence cant be used for EJBs.