Terms - App, ERPNExt, frappe, Module

trying to understand the logic as well as usage of terms a little better.
I hope someone here is able to clarify one the following:

  1. Is ERPNext an ‘App’ for frappe?
  2. how do you call a certain part of ERPNext (i.e. Accounts) then? is that a Module? or an App od ERPNext?
  3. if someone wants to add functionally to ERPNext is that another App or Module inside of ERPNext or another App for frappe?

I am trying to understand what certain terms precisely mean and maybe how they relate to each other.

This might be helpful: Session 1: Creating an App - YouTube