A notification is an important status message or warning. On a smartphone, you get one every time a text message comes in, an alarm goes off, a calendar appointment is imminent, or your battery is running low.
Phone apps use this mechanism, too. You get a message when your friends post updates on Facebook or Twitter. When your flight is two hours from takeoff. When a new Groupon discount becomes available. When your online Scrabble or chess partner makes a move.
It’s no different on your Mac. Mail and Messages might want to let you know that a new message has arrived. The Calendar app might want to remind you that an important meeting is about to start.
In macOS, you know when some app is trying to get your attention: A message bubble slides into view at the top right of your screen (Figure 11-1, top). Some of these alerts slide away again after 5 seconds; others require you to click a button, like Close, Snooze, or Show (which opens the program that’s waving its little hand in your face).
Only apps you got from the Mac App Store can tap into the Notification Center. Other programs will use whatever alert mechanisms they always have—pop-up dialog boxes, for example.
You’re also notified this way when one of your App Store apps has been automatically updated, or when that update requires you to quit the app or restart the Mac (and you’re offered a Later button).
All kinds of warnings—dying battery, failed Time Machine backups, disks ejected improperly—now ...