First, Windows 8 is the official product name for the next x86/64 editions of Windows.
For PCs and tablets powered by x86 processors (both 32 and 64 bit), we will have two editions: Windows 8 and Windows 8 Pro. For many consumers, Windows 8 will be the right choice. It will include all the features above plus an updated Windows Explorer, Task Manager, better multi-monitor support and the ability to switch languages on the fly (more details on this feature can be found in this blog post),which was previously only available in Enterprise/Ultimate editions of Windows. For China and a small set of select emerging markets, we will offer a local language-only edition of Windows 8.
Windows 8 Pro is designed to help tech enthusiasts and business/technical professionals obtain a broader set of Windows 8 technologies. It includes all the features in Windows 8 plus features for encryption, virtualization, PC management and domain connectivity. Windows Media Center will be available as an economical “media pack” add-on to Windows 8 Pro. If you are an enthusiast or you want to use your PC in a business environment, you will want Windows 8 Pro.
Windows RT is the newest member of the Windows family – also known as Windows on ARM or WOA, as we’ve referred to it previously. This single edition will only be available pre-installed on PCs and tablets powered by ARM processors and will help enable new thin and lightweight form factors with impressive battery life. Windows RT will include touch-optimized desktop versions of the new Microsoft Word, Excel, PowerPoint, and OneNote. For new apps, the focus for Windows RT is development on the new Windows runtime, or WinRT, which we unveiled in September and forms the foundation of a new generation of cloud-enabled, touch-enabled, web-connected apps of all kinds.
Windows 8 Enterprise for Software Assurance Agreements is the fourth edition
Full Story and edition comparison at source.