"Windows RT (formerly known as Windows on ARM) is an upcoming version of the Windows 8 operating system for ARM devices such as tablets. The RT acronym does not officially stand for anything.[1] It will officially only run software available through the Windows Store or included in Windows RT."
Windows 8 is a desktop/laptop/tablet OS aimed at home users
Windows 8 Pro is a desktop/laptop/tablet OS aimed at businesses/power users
Windows 8 Enterprise is a desktop/laptop/tablet OS aimed at large businesses (only available via volume license)
Windows RT is a tablet only OS aimed at the iPad/Android market (and can only be delivered via an OEM on a device) Like the iPad and Android it can only run apps available via the Microsoft App Store.
Surface RT is a tablet running Windows RT (so aimed at iPad/Android tablet owners).
Surface Pro (available early 2013) is a tablet running Windows 8 Pro and is more or less like an ultrabook with higher specs than the Surface RT. As it runs Windows 8 Pro it can run all apps that any other Windows 8 Pro computer can it is not limited to the App Store like the Surface RT is.
Thanks. Only after reading your response I understood.
Microsoft developed the Windows version for ARM. But then some big shot in Microsoft decided:
"No, you can't mention ARM in any name, we absolutely don't want to mention ARM anywhere in our propaganda or any other material. Our customers should not be aware that this thing like ARM processors exist. Intel promised to make something like ARM in just a year or two. Or we're going to make something. Or whatever, we just don't want to mention ARM. Verboten."
Underling1: "Oooh, how are we going to refer to the darn thing now?"
Underling2: "I know let's reuse the name that these developers use for the thing they produce, this WinRT thing. We'll call it Windows RT"
Kinda. It's confusing. Windows RT is supposedly "Windows 8 on ARM", a restricted version of Windows 8 where Win32 is not available to non-in-house developers (applications must be developed using WinRT a replacement API not to be confused with Windows RT as WinRT is also available to Windows 8 and Windows Phone 8); all applications must be distributed via the Windows Store; the fate of the Windows 8 desktop is unclear (though it won't be very useful if there); a bunch of software or capabilities are unavailable in RT (Windows Media Player, Media Center, Remote Desktop, Homegroup, ...).
The simplest way to think about it is probably to think that WindowsRT is to Windows 8 as iOS 9 (iPad) should be to OSX 11 should the current trends of the latter pair keep going
Windows RT is a version of Windows 8 built to run on ARM processors. "Metro" style apps can be compiled to run on Windows RT and downloaded from the Windows App Store. However traditional desktop software cannot be compiled for or installed on Windows RT.
"However traditional desktop software cannot be compiled for or installed on Windows RT."
What you meant to say that existing Windows apps cannot be installed or run on Windows RT. Technically they can be compiled to ARM, but there is no way you can install them. And Windows RT cannot emulate x86.
Windows 8 runs as Windows 8 and as Windows 8 RT. However, there's also WinRT which is something else:
WinRT == Windows Runtime
http://en.wikipedia.org/wiki/Windows_Runtime
"Not to be confused with Windows RT"
Now
"Windows RT (formerly known as Windows on ARM) is an upcoming version of the Windows 8 operating system for ARM devices such as tablets. The RT acronym does not officially stand for anything.[1] It will officially only run software available through the Windows Store or included in Windows RT."
Homework: try to explain that in less words. :)