For PCs and tablets powered by x86 processors (both 32 and 64 bit), we will have two editions of Windows to choose from, 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, Microsoft 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.
Do not confuse Windows RT with Windows Run Time.
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 and forms the foundation of a new generation of cloud-enabled, touch-enabled, web-connected apps of all kinds. For more details on WOA, this blog post shares more detail on how Windows 8 was built to run on the ARM architecture.
Source: Windows Blog Notifications
If you are still having plans on using Windows 7 and not sure on what version to use, read this article to understand the difference between Windows 7 Ultimate and Enterprise and other SKUs