The first three editions i.e Windows 8, Windows 8 Pro, Windows 8 Enterprise for PC and tablets users running x86/64 processors and will be on sale whereas Windows RT ill be exclusively for PCs and tablets rumming on the ARM platform and will not be sold but would rather ship built into the devices. According to Microsoft, all editions of Windows 9 offer a no-compromise experience, which has sort of become their slogan with Windows 8.
Windows 8 will be the base version meant for home users and is the equivalent of the Home Premium version of curren Windows 7. All basic features will be included in it.
Windows 8 Pro will have all the features of WIndows 8 but will add additional stuff such as encryption, virtualization, PC management and domain connectivity. You will be able to upgrade to this OS from Windows 7 Professional and Ultimate.
WIndows 8 Enterprise will include all features from Pro and also have “features for IT organization that enable PC management and deployment, advanced security, virtualization, new mobility scenarios, and much more.”
Windows RT (Real Time), or WOA as it was knows before, bill be exclusively for devices running on the ARM platform. It will be optimized for touch and will come with touch optimized version of Microsoft Office built-in, unlike the other three editions. However it will not allow the user to install any other application or the desktop side of the OS and the focus will be on using the Metro apps from the Windows Marketplace.