- There's the .Net framework, which includes WinForms and WPF/XAML for desktop apps (and ASP.Net for web apps).
- There's Silverlight, which is (was) a significant subset of WPF/XAML for browser plugin apps.
- There's the variant of .Net, based on Silverlight, For Windows Phone 7.X apps.
- There's WinRT, a native code reimplementation of alot of .Net and WPF/XAML exposed as a modernized COM API. It can be used from native code, .Net languages, and JS via WinJS. It is primarily for Windows Store Apps (ie: Metro, ie: tablet apps), though some of the non-UI bits can be used by desktop apps.
- There's whatever they're calling the .Net on Windows Phone 8 (WinPRT?), which is much closer to WinRT on Windows 8.
They're trying to unify those last two APIs. Unfortunately they are trying to unify the user experience across the related OSen as well.
Oh and there's some variation of WinRT for XBone too.
And there's also the .Net Compact framework, the .Net Micro Framework.
Hope that clears things up.