UINavigationController - iOS Programming: The Big Nerd Ranch Guide - page 354

UINavigationBar
The UINavigationBar isn't very interesting right now. A UINavigationBar
should display a descriptive title for the UIViewController that is currently on top of
the UINavigationController 's stack.
Every UIViewController has a navigationItem property of type UINaviga-
tionItem . However, unlike UINavigationBar , UINavigationItem is not a sub-
class of UIView , so it cannot appear on the screen. Instead, the navigation item supplies
the navigation bar with the content it needs to draw. When a UIViewController comes
to the top of a UINavigationController 's stack, the UINavigationBar uses the
UIViewController 's navigationItem to configure itself, as shown in Fig-
ure 11.13 .
Figure 11.13 UINavigationItem
By default, a UINavigationItem is empty. At the most basic level, a UINaviga-
tionItem has a simple title string. When a UIViewController is moved to the
top of the navigation stack and its navigationItem has a valid string for its title
property, the navigation bar will display that string ( Figure 11.14 ) .
Figure 11.14 UINavigationItem with title