There were six versions of Windows 7 and that seemed to have been a bit too much for users to understand and identify, based on their needs. So Microsoft is releasing their next OS as just three versions. Firstly, you have the basic Windows 8, that works fine with the basic features and a quick language switch, as and when you find it necessary. Then, you have the Windows 8 Pro that will use a fair bit of encryption to protect files better and a Media Centre to satisfy various taste buds. Finally, there will also be the Microsoft RT (runtime) that will work on systems using ARM processors.

The ‘RT’ rendition will come pre-installed on ARM-running systems and will facilitate touch versions of the MS Office suite and OneNote.
CNet