A basic push notification is a message or alert delivered by a centralized server to an endpoint device.
Push allows an installed 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.
For iOS devices, push notifications allow apps to display a number or 'badge' on the app's icon (for non-newsstand applications). For example, the Apple Mail icon will show the number four when we have four unread messages.
Push Notifications can also be seen on the lock screen or displayed in the notification center, which can be opened by swiping a finger from the top of the iPad downward toward the bottom of the iPad. The display of push notifications can differ across Android devices and there is no Android standard, most push providers write their own interface for displaying push on Android.
Unless you have opted for provisional pushes your users will be prompted to allow or not allow push notifications once they download your app.
For Android devices, when you tell the system to issue a notification, it first appears as an icon in the notification area. To see the details of the notification, the user opens the notification drawer. Both the notification area and the notification drawer are system-controlled areas that the user can view at any time.