iPhone Push Notifications – Push notifications are a technique used by apps to alert iPhone and iPod touch owners to content updates, messages, and other events within an app that users may want to be aware of.
Push notifications are only available once you install an app that uses them. When such an app is installed, a Notifications menu is added to the main Settings screen of the device. On that screen, users can turn push notifications on or off for their entire device, or control notification settings for individual apps.
When an event occurs that causes a given app to generate a notification, the notification is pushed to the user by one of three options: sound, onscreen alert, or badge (the name Apple uses for added the red, circular number icon to an app icon, a la the Mail app showing how many new messages you have). Most apps that support push notifications allow users to select which of the three notifications (or none) they prefer to use to make them aware of updates.