WinRT

(WINdows RunTime) The Windows programming interface (API) that superseded Win32 and debuted with Windows 8. WinRT allows developers to create safe "sandboxed" touchscreen applications available from the Microsoft Store. WinRT apps support both the x86 and ARM architectures and multiple programming languages, including C/C++, C#, Visual Basic and JavaScript. WinRT was augmented by the Universal Windows Platform (see UWP). See Win32.

WinRT vs. Windows RT
WinRT is the API, while Windows RT was the operating system for ARM-based tablets when Windows 8 was introduced. Windows RT was the first operating system that required developers to employ the WinRT APIs. See Windows RT, Windows 8 and Windows 8.1.

A Peculiar Name
The "Runtime" in WinRT is an odd name for a programming interface (API) because every API represents operating system modules that must be "running" in order to work. The common convention for the term "runtime" is a third-party, non-OS module that must be loaded; for example, a Java interpreter. Strange naming is however very common in the industry (see naming fiascos).

The WinRT API

When Windows 8 was introduced, developers had to use the WinRT API in order to create the new tablet-style apps.