JBoss is essentially a EJB container, and Tomcat is a servlet container. The good thing or the confusing part is that JBoss has a pluggable architect as a result you can run servlet container like Tomcat within JBoss.
JBoss was originally just an EJB/JMS container. JBoss now comes bundled with Tomcat (a JSP/Servlet container). So, JBoss is running the show and it runs Tomcat as a managed service. Overall, it's a seamless integration of products that work well together.
Originally posted by Scott Selikoff: Wasn't installing Tomcat an extra step in older versions of JBoss? Its been so long since I've done it though.
Quite possibly. I remember back to either 3.0 or 3.1 and you could download either a Tomcat version or a Jetty version. They were/are plugable components though, so you should be able to remove one and have a version of JBoss without a web container...(if you really wanted to)