Read original post by Andrei Marukovich at LunarFrog
Windows Store apps use Flyouts for Login and Settings forms, About dialogs and for displaying other temporary information. While HTML5/JS developers experience the comfort of using native WinJS.UI.Flyout control, XAML developers have to use third-party controls.
Prism for Windows Store provides infrastructure for displaying and managing Flyout dialogs in a simple way.