The four versions of Windows 8 break down as follows:
Windows 8. This version targets traditional Intel-compatible x86/x64 PCs, supports upgrading from Windows 7 Starter, Home Basic, or Home Premium, and provides all of the basic Windows 8 features, including the Start screen with semantic zoom, live tiles, Windows Store, core apps (Mail, Calendar, People, Messaging, Photos, SkyDrive, Reader, Music, and Video), Internet Explore 10, connected standby, Microsoft account integration, the Windows desktop, and much more.
Windows 8 Pro. This version targets traditional Intel-compatible x86/x64 PCs and supports upgrading from Windows 7 Starter, Home Basic, Home Premium, Professional, or Ultimate. It provides all of the features from the basic Windows 8 version and adds BitLocker and BitLocker To Go, Boot from VHD, Client Hyper-V, domain join, Encrypting File System (EFS), Group Policy, and Remote Desktop (host).
Windows 8 Enterprise. This version includes all the features in Windows 8 Pro “plus features for IT organization that enable PC management and deployment, advanced security, virtualization, new mobility scenarios, and much more.” This version will be provided only to those corporate customers with Software Assurance agreements.
Windows 8 RT. This version will only be available pre-installed on ARM-based PCs and tablets, which will feature “new thin and lightweight form factors with impressive battery life,” Microsoft says. Previously called Windows On ARM, or WOA, Windows 8 RT will include touch-optimized desktop versions of the new Microsoft Word, Excel, PowerPoint, and OneNote 15, and support device encryption capabilities. It will lack some features, however, including Storage Spaces, Windows Media Player, and all of the features that are unique to Windows 8 Pro.