If you're a system admin login to your instance and go to System Definition > Tables and search for the Department table. When you find it click in to the detail and click the "Show schema map" link at the bottom of the list of columns. This will show a part of the hierarchy that you're asking about.
For better or worse groups don't really play into this organizational hierarchy per-se. Users have a one-to-one relationship with the department and a one-to-many relationship with groups. If you understand what I just said then the picture should start to form for you. If not please let me know and I'll try to be more specific.
