Apple Push Notification service (APNs) is a cloud service that allows approved third-party apps installed on Apple devices to send push notifications from a remote server to users over a secure connection. For example, a newstand app might use APNs to send a text alert to an iPhone user about a breaking news story. It might also apply a numerical badge to the app’s icon, informing the user how many stories are waiting.
Push notifications are popular on mobile devices because they conserve battery life. With pull notifications, mobile apps are required to continually poll the developer's server, connecting every few minutes to determine if new information is available. With push notifications, however, the cloud service acts on behalf of the app and only connects to the mobile device when there are new notifications. If the device is powered on but the app is not running, the service will still forward the notification. If the device is powered off when a notification is sent, the service will hold on to the message and try again later.
To receive APNs push notifications, the app must be configured properly and registered with the Apple Push Notification Service (APNs). The service delivers notifications through an application programming interface (API) that is included in all iOS and Mac OS X devices. Apple first introduced APNs in June 2009 with iOS 3 for the iPhone. A Notification Center, which was first included in 2012 with the release of iOS 5, allows users to manage and read notifications in one place.