Terms you'll need to understand:
Web app deployment descriptor
Context and attribute listener
Techniques you'll need to master:
Retrieve servlet context-initialization parameters.
Use a servlet context listener.
Use a servlet context attribute listener.
Use a session attribute listener.
Distinguish the behavior of listeners in a distributable.
You can see how Tomcat employs an architecture that implements Sun's specifications carefully. It is hard to understand context, though, if you don't know the overall architecture.
When you see the word application ...