How are Apache HTTP Server and Apache Tomcat related? (If at all)

Question

I currently have Apache httpd running on a production Ubuntu VPS server. I write php scripts.

I'm interested in learning Java and I was wondering how I would go about writing some server-side Java to work on my current set-up.

How are Apache Tomcat and Apache HTTP Server related to each other?

Can Tomcat be a module of httpd?

Or are they simply just two very different projects that happen to be steered by the same organisation (Apache Software Foundation)?