There are lot of confusion and questions around Windows 8, WinRT..
Let us break this down here.
Windows 8 Core OS Services is the core kernel which is same as in Windows 7 with rewrite in some areas and complete improvements in many areas.
Desktop Apps runs only on x86 or x64. It will not run in ARM devices where Windows RT only is present. However it will work in Surface tablet with Intel Processors where the Core is the full Windows 8 OS.
WinRT API’s which is sitting on top of WinRT Core is similar like .NET which is sitting in top of OS.
Key feature of WinRT is the language projection & metadata which allows the WinRT API to be consumed natively by the languages stack(.NET for Win RT, WinJS) listed above. Note WinJS is meant only for WinRT and it will run by itself in a web stack.
Apps built on top of the WinRT API are the Windows Apps which run in Windows 8 OS and in Windows 8 Tablets.
- 5 Points Developers Need to Know about Windows RT (mobile.dzone.com)