Push notification allows an app to notify you of new messages or events without the need to actually open the application, similar to how a text message will make a sound and pop up on your screen. This is a great way for apps to interact with us in the background, whether it be a game notifying us of some event occurring in our game world or simply the iPad's mail application beeping as a new message appears in our inbox.
In addition to pushing messages to the screen, push notification allows apps to display a number or 'badge' on the app's icon. For example, the Mail icon will show the number four when we have four unread messages.
Push notification was added to iOS in the 3.0 update.