Have you ever wondered how tab bar controllers and navigation controllers do their work? Even though it may seem as if UITabBarController and UINavigationController are magical classes, they are nothing more than UIViewController subclasses.
What have these classes in common? Both classes allow you to insert custom content in the form of one or more view controllers. A navigation controller, for example, manages a stack of view controllers. You can push and pop view controllers onto and from a navigation stack. The same is true for a tab bar controller. It manages an ordered list of view controllers accessible through a tab bar at the bottom.
Read this article on the blog.