By Steve Rowe:
The Windows Runtime is exposed using API metadata (.winmd files). This is the same format used by the .NET framework (Ecma-335). The underlying binary contract makes it easy for you to access the Windows Runtime APIs directly in the development language of your choice. The shape and structure of the Windows Runtime APIs can be understood by both static languages such as C# and dynamic languages such as JavaScript. IntelliSense is available in JavaScript, C#, Visual Basic, and C++.
In short, Windows Runtime is a new set of libraries exposing Windows functionality and available to JavaScript/C#/VB/C++. Each language has been made to understand and be able to call them directly rather than having to go through some thunking layer.
Silverlight and WPF are flavors of XAML that run on the CLR. Among other functionality, Windows Runtime exposes a version of XAML very similar to Silverlight, but does so in a native way, not via the CLR. It can be accessed from the CLR, but also from C++.
