What are the roles of Tomcat, Jboss and jdk7 in server construction?

I have an old web application. It is implemented with a stack of technologies including these: apache-tomcat-7, jboss-as-7.1.1 and jdk7. I need to define for what each of them is responsible for.

So far I have googled that Tomcat and JBoss can be interchangeable. On the other hand, I have discovered that Tomcat is simple a container then Jboss is a full featured server. I also see that often questions about tomcat and JBoss are stated as one against another. But in my case, I have a document that says that both JBoss and Tomcat are used for this app.

What might be the roles of this software in server building when they both are present?