Tomcat is an Engine Servlet
JBoss is an Application Server
The differences or advantages in using JBoss Web Server and not Tomcat is if you must deploy of a web-app ENTERPRISE (example to use EJB).
Example Tomcat not support EJB.
erasmarciano, the question was what is the different between JBoss Web Server (not JBoss Application Server) and Tomcat.
xcenogar, as far as I can see, there are two main difference. You already pointed out one - JBoss Web Server comes with the APR libraries. The second is that JBoss Web Server also has PHP support (notice the phpservlet JAR file in the lib directory). On the other hand, I notice that Tomcat 6.0_18 has built-in clustering support (note the tribes jar in the lib directory).