WinBeta exclusively revealed not too long ago that a Notification Center, along with many other features would be arriving with Windows 9 (Threshold) in 2015. Today, more details regarding the Notification Center have been revealed courtesy of Neowin, who explain how the Notification Center will work on desktops.
According to Neowin, the Notification Center (in most recent builds) is a fixed window which is displayed in the bottom right hand side of the screen, and works like you'd expect it to. When an app has a notification for you, it'll be present in the Notification Center until tapped on or dismissed, much like Windows Phone.
Considering this info focuses on the desktop side of things, the Notification Center can be accessed from the System Tray. We're still not sure how the Notification Center will be accessed with tablets, we're guessing it'll most likely be a swipe down from the top of the screen.
In the most recent builds of Threshold, the Notification Center is very simple, offering no real features other than being a home for notifications, unlike its Windows Phone counterpart which includes an Action Center. We assume Microsoft will add more features to the Notification Center in good time, but at least we're finally seeing the beginnings of what will be a fantastic way to organize notifications.