Windows 8 Pro is a desktop/laptop/tablet OS aimed at businesses/power users
Windows 8 Enterprise is a desktop/laptop/tablet OS aimed at large businesses (only available via volume license)
Windows RT is a tablet only OS aimed at the iPad/Android market (and can only be delivered via an OEM on a device) Like the iPad and Android it can only run apps available via the Microsoft App Store.
Surface RT is a tablet running Windows RT (so aimed at iPad/Android tablet owners).
Surface Pro (available early 2013) is a tablet running Windows 8 Pro and is more or less like an ultrabook with higher specs than the Surface RT. As it runs Windows 8 Pro it can run all apps that any other Windows 8 Pro computer can it is not limited to the App Store like the Surface RT is.
WinRT is the new Windows Run Time. Read more at http://en.wikipedia.org/wiki/WinRT as it explains it better than I can in one sentence.