Tomcat is, in this configuration, a stand-alone TCP/IP server that communicates with Apache via mod_jk. It can also act as a simple HTTP server in it's own right, and there are ways of getting it to run within the Apache process as well. This document does not cover that however. For a better description of the whole Apache->mod_jk->Tomcat process, see http://www.ccl.net/cca/software/UNIX/apache/tomcatfaq.shtml.