Most home computer users will likely be fine with Windows 8. Windows 8 Pro is designed for the "enthusiast" and the business environment. It includes features for encryption, virtualization, PC management and domain connectivity. In addition, Windows Media Center will be available as an economical “media pack” add-on to Windows 8 Pro. .
There will also be Windows RT which is Windows on ARM or WOA. Windows RT will only be available pre-installed on PCs and tablets powered by ARM processors. It will help enable new thin and lightweight form factors with impressive battery life. As explained by Brandon LeBlanc in the Windows Blog,
"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. For more details on WOA, we suggest reading this blog post which shares more detail on how we have been building Windows 8 to run on the ARM architecture."Although any of the Windows 7 versions can upgrade to either Windows 8 or Windows 8 Pro, Windows 7 Ultimate an Windows 7 Pro can only upgrade to Windows 8 Pro.
The chart at Announcing the Windows 8 Editions breaks down a starter list of key features by edition and upgrade path.