Over the past fifteen months, we have been referring to Microsoft's upcoming version of Windows 8 specifically catered for ARM-based devices as "Windows 8 ARM Edition." Announced during Microsoft's CES 2011 keynote, the ARM-based version of Microsoft's next OS has been under development for quite some time and started seeding to developers only two months ago in February.
On Monday, Microsoft formally announced four versions of Windows 8 that it will be officially launching on the consumer and enterprise markets. The first three versions will be physically available in retail / online shelves, and they are simply called "Windows 8," "Windows 8 Pro" and "Windows 8 Enterprise."
Microsoft Blog Post: Announcing the Windows 8 Editions
Unlike Windows 8 and Windows 8 Pro, Windows RT won't be upgradeable from any edition of Windows 7, obviously, but it also won't get Windows Media Player or Storage Spaces. What Windows 8 RT does give you is Device Encryption and Microsoft Office already bundled with the OS, neither of which is available for Windows 8 and Windows 8 Pro.
Economically speaking, however, Windows 8 RT is expected to be one of the most cost-efficient patforms available when it goes massively public around the timeframe of Mobile World Congress 2013. Unfortunately, the release date had been pushed back from Q3 2012, to an announcement during Mobile World Congress 2013, and finally availability beginning Q2 2013, according to several highly-ranked Microsoft executives.
Microsoft's full blog post announcing all Windows 8 product editions can be found here.