UIApplicationDelegate-> UIApplicationDelegate is a protocol which consists some predefine methods. These methods are called by singleton UIApplication object.

UIApplication Object-> UiApplication Object is the object of UIApplication Class which perform it’s functions at the time of launch. Basically it ensures that your application is working properly with the  system and with other applications. It control and coordinate Apps running in IOS. When an app launches it calls UIApplicationMain function which create a singleton UIApp object.  Now you can access the methods of shared App Class .

Singleton- Singleton means making a public global object which is used as a single object through the project.