After years of confusing consumers with multiple, slightly different versions of the same operating system, Microsoft announced that Windows 8 will come in only four versions, they are:

Windows 8 (Home use version)
Windows 8 Pro (for business)
Windows RT (for devices running ARM chips)
Windows 8 Enterprise

However, for most people buying an operating system for a traditional desktop or laptop, the choice will be between just two versions. The version called simply "Windows 8" is designed for home users.

Windows 8 Pro is for business users and includes features for encrypting a file system, virtualization, and domain management.

Windows RT is the new name for what had been called Windows on ARM. You won't be able to purchase it on its own; it'll come preinstalled on PCs and tablets that run ARM processors. Windows RT won't be able to run traditional X86/64 desktop software. Instead, it'll run touch-oriented apps based on Windows Runtime (or WinRT), Microsoft's programming model for mobile apps. Apps for the touch-oriented Metro interface are built using Windows Runtime.
The windows RT will come with special touch-oriented versions of Microsoft Word, Excel, PowerPoint, and OneNote.

Windows 8 Enterprise includes all the features of Windows 8 Pro plus features for IT organization that enable PC management and deployment, advanced security, virtualization, new mobility scenarios, and much more.

http://www.luqnet.com/2012/04/new-versions-of-windows-8.html