Windows 8 and Windows 8 Pro are for x86/x64 architectures.
Windows RT is for ARM architectures.
Besides the fact that windows RT will only be able to run in ARM architectures, i will NOT be sold separately AFAIK. You can only get it "bundled" with a device, like Microsoft Surface.
I also read that win RT WILL always come with office preinstaled, though it will be a "reduced" version of office, office 2013-RT (Word, Excel, Powerpoint, OneNote); which also will NOT let run macros.
In general, RT does not only mean that it is intended for ARM architectures, but also it is a DIFFERENT version of windows, with some limitations, as for example not being able to install any program that hasnt been designes specifically for win RT, and you will only be able to get apps from the new windows store.
Think of windows RT as similar to iOS or Android, and Windows 8/Pro as the well know "desktop" version of windows, intended for desktops and notebooks.
I'm afraid that a LOT of users will get confused and will start to get tablets with windows RT expecting they can do exactly the same they can with their regular notebook/desktop. They will NOT.