Windows 8 introduces two app environments - desktop apps and Metro apps. Metro apps are optimized for tablets and touch, use new APIs like WinRT, and are distributed through the Windows Store. Desktop apps can still be developed with existing technologies like WPF, but can only be sideloaded and not distributed through the Store. The development stack includes options for HTML, XAML, and native code apps across Windows 8, Windows RT, and Windows desktop through a common WinRT API layer.