whats the use of UIApplicationDidEnterBackgroundNotification
in iPhone app or how we can take benifit from it
This notification means the user "quit" your app on an iPhone 4 - It happens when a phone call or text message comes in and user accepts the interruption (answers/replies), or when the user has pressed the Home button.
I found this link on SO that shows the interaction between all states, and the appropriate notifications: http://www.drobnik.com/touch/2010/07/understanding-ios-4-backgrounding-and-delegate-messaging/
To make use of this notification you can implement applicationDidEnterBackground as @Antwan suggested (in your UIApplicationDelegate class - that's the main class).
Alternatively you could set up a notification handler wherever you want/need in your code:
[[NSNotificationCenter defaultCenter] addObserver: self
selector: @selector(handleEnteredBackground:)
name: UIApplicationDidEnterBackgroundNotification
object: nil];
Good luck!
Oded.