Windows RT or WinRT was an operating system intended to be used on devices with ARM processors, like tablets. It is only capable of running software from the Microsoft Store, as well as any software that is bundled and installed with Windows RT initially. The operating system was never available for purchase separate from a device, but was available to manufacturers and sold on the Microsoft Surface.
While Windows RT shared some similarities with Windows 8, including some code base, there were some differences. Windows RT did not include the Windows Media Player, which is standard in Windows 8. However, it will include the Microsoft Office 2013 RT suite for free, whereas, for Windows 8, the Office suite must be purchased separately. Also, developers must use the Windows Runtime to write software to be used on Windows RT devices.
Note: Windows 8 software or software for previous versions of Windows cannot be installed or run on Windows RT.
As of January 2015 Microsoft no longer makes or sells any devices that use Windows RT. However, Microsoft will continue support for Windows RT 8.1 until January 9, 2018.