Read original post by Andrei Marukovich at Canadian Developer Connection Blog
WinRT is a set of modern, object-oriented APIs introduced in Windows 8. WinRT is built on top of Win32 and COM and is designed to provide interoperability for the various languages using metadata. In addition to the previously available Windows and .NET APIs, now exposed in a cleaner, streamlined way, WinRT provides an access to all the new features of Windows 8.
Some WinRT APIs are closely coupled with the new Windows Store app model while others can safely be used for desktop application development. For Windows desktop applications, using the WinRT APIs enables simple ways to access GPS and NFC devices, the accelerometer, orientation, light, and other sensors. Moreover, some WinRT classes are only available for desktop apps, like PackageManager.