The Start Screen is a redesign of the Windows user experience, based on changing hardware and updated to reflect user's habits based on Microsoft's telemetric data. The Start Screen combines the functionality of the Superbar (taskbar in Windows-7), the Desktop, the Notification Area, and Start Menu.
The old Desktop, Superbar, and Notification Area still exist, along with a trimmed down version of the old Start Menu.
The Tiles can not only launch applications, but it can also display live content, like an RSS feed, a picture slideshow or a dashboard. The Tiles provide a larger hit area for touch interface, which are becoming more common in mobile devices, along with taking advantage of the expanding displays in desktop machines. Tiles may also feature badges, which are glyph overlays that display a new notification like 10 unread messages.
Notification messages have been replaced with Toasts: which are the primary method for background processes to communicate to the user. Toasts look like a rectangle that slide in from the bottom corner and not only notify users, but can also present users with optional actions to choose from.