WinRT is the API that is used to write the Store (Metro) apps - it stands for Windows RunTime. On Windows 8 the WinRT API is supported alongside the traditional Win32/64 API that is used for desktop apps; on Windows RT it is the only officially supported API for 3rd-party developers - Win32 is only officially available to Microsoft.
Originally Posted by kashk5
AFAIK Windows RT got its somewhat odd name from WinRT, but the two are most definitely *not* the same. The English speak English, and the Americans speak English, but Americans are not English (at least not recently).
Anyway, when people talk about Outlook RT, or more generally any other "RT" app, what they're usually talking about is a version written for WinRT and hence capable of running on Windows RT. But most apps written for WinRT will run on both Windows RT and Windows 8 (the exceptions are some games that are compiled to processor-specific native code instead of the processor-independent CLR).
Back to the point I was originally making earlier in this thread. While microsoft may well be working on a version of Outlook for the WinRT, there's no guarantee that they will release it for Windows RT. They have the option of only releasing it for x86 machines; and since the people most interested in a Windows Store style Outlook app are the same ones that also need all the other enterprise stuff that's missing from Windows RT but present in the higher-end x86 versions of windows, I wouldn't be surprised if that is the approach Microsoft takes.