Windows RT is a special edition of Windows 8. It runs on ARM and you’ll find it alongside Intel x86 machines in stores, but you’ll be surprised just how much Windows RT differs from the Windows you know.
Windows RT is so different that Microsoft has told Mozilla Windows RT “isn’t Windows anymore.” If you’re looking to buy a Windows system in stores, you should know the difference between Windows RT and the other editions of Windows 8.
ARM vs. x86 - Windows RT was known in development as Windows on ARM, or WOA. It's essentially a port of Windows from Intel x86 processors to ARM processors. x86 processors are what you'll find in standard laptops and desktops today, while the vast majority of smartphones and tablets use ARM processors.
Because it's a port of Windows to a different architecture, it doesn't support legacy software – that is, all software written for Windows already. It's known as Windows RT because it only supports applications written for Windows Runtime or WinRT (yes, "WinRT" refers to the runtime that works on both architectures, while "Windows RT" refers to the operating system that only works on ARM). You probably know Windows Runtime applications as "Metro apps."