App and Root are two different objects. root is the root window into which all other widgets go. It is an instance of the class Tk, and every tkinter application must have exactly one instance of this class. app is an instance of the class App, which is a subclass of Frame. A frame is typically used as a container for other widgets, in this case the widgets that make up the app (except for the root window). This frame is packed inside the root window. app and root are two completely different things. root is a container for app.
