One of the new core APIs that will be included with Windows 8 is called WinRT, and it is primarily used to create Metro style apps along with C/C++, .NET or JavaScript. With the release of Windows 8, Microsoft will be officially dumping the Win32 API.
By using WinRT, rather than the Win32 API, developers can create apps that feature hardware acceleration and advanced power management functionality. Another benefit is that existing Silverlight and WPF applications can be moved over to the new “Native XAML” libraries without any additional expenditure or excessive development time.