I don't understand what's going on in the docker-compose example.

Here are my questions about it:

- what is an actual service i'd like to run? like any app? Deluge?
- is the web server just a proxy or way to access the service I'm running? 
- what if the app is supposed to run on a certain port?
- what does running two services look like? 
- what does running an external service look like?