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. :)