first: Apache Tomcat is not just a webserver. Many people would in fact call it an application server.
Websphere, iPlanet, SunONE,
JBoss -> These things are usually called app servers, and what they really mean is "it does
EJB's".
Tomcat is not quite a "full-blown" app server, because it only does servlets/jsps and not EJBs.
a plain old "web server" (like Apache), serves only HTML pages (as long as it doesn't have any modules).