Notifications in iOS

In iOS applications there is a concept of a “Notification Center”. It is a singleton object that allows for objects to be notified of events occurring. It allows us to satisfy the goal of communicating between a controller and an arbitrary object with a low level of coupling. The basic concept of this pattern is that a controller uses a key (notification name) in order to allow other objects to hear about special events occurring within the controller. Then unbeknown to the controller, other objects (observers) can react to the notification events by registering for notifications with the same key.
Pros

• Easy to implement, with not many lines of code.
• Can easily have multiple objects reacting to the same notification being posted.
• Controller can pass in a context (dictionary) object with custom information (userInfo) related to the notification being posted.

Cons

• No compile time to checks to ensure that notifications are correctly handled by observers.
• Required to un-register with the notification center if your previously registered object is deallocated.
• Not very traceable. Attempting to debug issues related to application flow and control can be very difficult.
• Third party object required to manage the link between controllers and observer objects.
• Notification Names, and UserInfo dictionary keys need to be known by both the observers and the controllers. If these are not defined in a common place, they can very easily become out of sync.
• No ability for the controller to get any information back from an observer after a notification is posted.

Source:
http://blog.shinetech.com/2011/06/14/delegation-notification-and-observation/