When the user taps on the app icon, the app briefly goes through a transitional state of UIApplicationState.Inactive on its way to becoming UIApplicationState.Active. This is where the app gets itself ready to display to the user.

When the app is open, the application state is UIApplicationState.Active.

If the user presses the home button, and is returned to the springboard (home screen), or the application is interrupted by something, such as a phone call, the application state transitions back to UIApplicationState.Inactive.

For the application state of the app to become UIApplicationState.Background the application would have to register for a background process.

Reference: http://stackoverflow.com/questions/7937400/need-clarification-about-uiapplicationstate