Windows 8 is a core edition intended for the average home user.
Windows RT is a version of Windows 8 for the Surface tablets.
Windows 8 Pro is targeted at enthusiast and business markets. It adds the ability to run Hyper-V virtual machines.
Windows 8 Pro with Media Center includes the Windows Media Center.
Windows 8 Enterprise is designed for large organizations. It has the same feature set as Pro (excluding Media Center) but supports creating Windows To Go portable workspaces.