The EPUR1 What Runs page is a walkthrough of exactly what instances of what components are run (including what pre-existing services are available). It starts from the operator's view of starting with nothing but downloading the epumgmt tool. This section gives you a feel for what the system actually is from an operational point of view.
Also temporary (?) home for: R1 What Runs
The EPUR1 Packages page is the current idea about what the package/repository/dependency layout should look like.