Описание тега android-notifications
A status notification adds an icon to the system's status bar (with an optional ticker-text message) and a notification message in the notifications window.
A notification is a user interface element that you display outside your app's normal UI to indicate that an event has occurred. Users can choose to view the notification while using other apps and respond to it when it's convenient for them.
Think of notifications as a news channel that alerts the user to important events as they happen or a log that chronicles events while the user is not paying attention. The user can expand the notification bar and by selecting the notification the user can trigger another activity. Notifications in Android are represented by the Notification class. To create notifications you use the NotificationManager class which can be received from the Activity via the getSystemService() method.