When you receive a notification, a small floating window appears in the upper right corner of the Mac's display, containing the notification. Notifications come in two forms: banners and alerts. If it's a banner, the message appears for a few seconds, then disappears from view; alerts require you to manually acknowledge them before they disappear. Clicking on a banner or alert will send you directly to the relevant application.
So who decides what's an alert and what's a banner?
You do. Using the new Notifications pane in the System Preferences app, you can set on a per-app basis whether you want your notifications to appear as five-second banners or alerts that remain visible until you click them.
That preferences pane is also where you can set which apps appear within Notification Center--think of it as the OS X version of the Notifications submenu in iOS's Settings app. By default, Calendar, Reminders, Game Center, Safari, Mail, and Messages are configured to send notifications. You can also choose how many reminders appear per application--1, 5, 10, or 20 Items--and you can choose to display a badge on each application's Dock icon when a notification from that app is received. You can additionally configure notifications so that a sound is played when a notification is received.
What if I don't see a banner notification before it disappears?
If a banner notification disappears before you can get to it, you can bring up Notification Center by clicking on the Notification Center icon in the menu bar (the double circle in the upper right corner of the menu bar), entering a keyboard shortcut, or using a two-finger swipe. The desktop will shift to the left, displaying all current notifications in a dark gray column, sorted by app along the right edge of your screen. When you click on a notification in this list, the corresponding application will open and display the related item. For example, if you click on a Mail subject heading, Mail will open and display that message.
What's the multitouch gesture to open Notification Center?
Starting at the far right edge of the trackpad, swipe with two fingers to the left (as if you were pulling something away from the right side of the screen). You can also assign a keyboard shortcut to show and hide Notification Center. You can do this within System Preferences -> Keyboard -> Keyboard Shortcuts.
Can you hide the Notification Center menu bar icon?
Not that we can tell. But we expect that might change.
What is Gatekeeper? How will it work?