From the Apache HTTP Server Project web page (http://httpd.apache.org/)
The Apache HTTP Server Project is an effort to develop and maintain an open-source HTTP server for modern operating systems including UNIX and Windows NT. The goal of this project is to provide a secure, efficient and extensible server that provides HTTP services in sync with the current HTTP standards.
From the Apache Tomcat web page (http://jakarta.apache.org/tomcat/index.html):
Apache Tomcat is the servlet container that is used in the official Reference Implementation for the Java Servlet and JavaServer Pages technologies. The Java Servlet and JavaServer Pages specifications are developed by Sun under the Java Community Process.
From the Apache Jakarta Tomcat Connector page (http://jakarta.apache.org/tomcat/connectors-doc/faq.html)
JK is a project covering web-servers to Tomcat connectors, whereas mod_jk is the Apache module developed in JK.
This HOWTO is intended as a complete, step-by-step guide to setting up Apache Tomcat servlet container in conjuction with the Apache HTTP Server configured to pass requests through mod_jk.
If you have JRun installed, either by itself or in conjunction with a stand-alone ColdFusion installation, JRun will presume to try to compile all JSP files (Even if you are running Standard or Professional edition which will give a License Exception). This includes all JSP files under your Tomcat webapps. To prevent this, in your Apache configuration under the JRun settings change Ignoresuffixmap to true and delete .jsp and .jws from the AddHandler entry. This will prevent JSPs from running under JRun, so if you are running JRun and Tomcat, decide which application server you want to handle your JSP files and configure your setup accordingly.