WinRT, distilled to its simplest definition, is a Windows API that sits directly on top of the core Windows 8 services. As such, it is actually a direct peer of the previous Win32 API. Microsoft invested in a new runtime library for a few different reasons. For one, Win32 APIs weren’t the easiest to access and develop against from a .NET perspective. The impedance mismatch between the .NET Framework surface and the Win32 API/COM surface made for sometimes confusing, and sometimes impossible, development tasks.
Figure 23.7 shows the traditional “layer cake” architecture diagram, clearly demonstrating where the WinRT sits in relationship to the other parts of the OS and the development platform.