Legacy software written for Windows 7 and prior targeted the traditional Windows API, often referred to as Win32. They resulting apps would be a combination of an executable, assets and libraries. The Windows 8 operating system introduces a new type of application, called a Windows Store app. The new architecture is called Windows Runtime, or WinRT for short. Windows Store apps are written exclusively against this new WinRT architecture.
Windows 7 and prior only had one mode of interacting with software, launching the application’s executable from the desktop or file path. Windows 8 now defaults to a Windows Runtime start screen, where Windows Store apps are presented and can be accessed through their respective Live Tiles. An application’s Live Tile is registered automatically when that app is installed from the Windows Store. If a user needs to access legacy software that hasn’t been migrated to the new Windows Store format, they can still get to the desktop from the Start Screen.
Windows desktops and laptops have traditionally run on Intel processors, referred to frequently as x86 or x64 machines. However, in parallel with the release of Windows 8, an optimized version of the operating system designed for mobile ARM based devices was released called Windows RT. Windows RT devices, however, will not run legacy Win32 software, nor can you develop Win32 software for content.
So unlike a Windows 8 computer, which is optimized for the Windows Store apps written against WinRT but still provides access to Win32 software from the desktop, Windows RT devices do not.