Source: http://developer.android.com/guide/topics/ui/notifiers/notifications.html
Heads-up Notifications
Figure 3. Fullscreen activity showing a heads-up notification
With Android 5.0 (API level 21), notifications can appear in a small floating window (also called a heads-up notification) when the device is active (that is, the device is unlocked and its screen is on). These notifications appear similar to the compact form of your notification, except that the heads-up notification also shows action buttons. Users can act on, or dismiss, a heads-up notification without leaving the current app.
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