UINavigationController - iOS Programming: The Big Nerd Ranch Guide - page 335

Like UITabBarController , UINavigationController has a viewCon-
trollers array. The root view controller is the first object in the array. As more view
controllers and pushed onto the stack, they are added to the end of this array. Thus, the
last view controller in the array is the top of the stack. UINavigationController 's
topViewController property keeps a pointer to the top of the stack.
UINavigationController is a subclass of UIViewController , so it has a
view of its own. Its view always has two subviews: a UINavigationBar and the
view of topViewController ( Figure 11.3 ) . You can install a navigation controller
as the rootViewController controller of the window to insert its view as a subview
of the window.
Figure 11.3 A UINavigationController's view