Following are the client versions of Windows 8 and 8.1. When upgrading to Windows 8 from an existing Windows operating system via download, Windows 8 retains the same word size. For example, a 32-bit Windows 7 machine will be upgraded to a 32-bit Windows 8 operating system. See Windows 8
The standard Windows 8 version, which runs all previous versions of Windows.
Windows 8 Pro
The Pro version adds data protection via encryption, remote desktop capability (access your PC remotely) and domain connectivity in company and school networks.
Windows 8 Enterprise
Volume purchases of Windows 8 Pro sold to large organizations.
Windows 8 Media Center
Available as an upgrade package.
Although Windows RT looks and feels just like Windows 8, RT is a tablet-only version of Windows 8 that runs apps written only to run on ARM CPUs rather than x86 CPUs. RT comes pre-installed on tablets. See Windows RT