WPF
From RemObjects Software
The Windows Presentation Foundation is a set of APIs included with .NET 3.0 that expose a vector and raster based GUI framework.
WPF was introduced with Windows Vista though it also runs on Windows XP SP2. WPF is marketed as the next GUI framework to replace WinForms.
WPF uses XAML files instead of code to describe the GUI, where an XAML file is a human readable XML file, allowing for a clean separation between business logic code and UI. WPF applications can run standalone and inside a browser, like WinForms applications.
Silverlight
Silverlight is a subset of WPF, a cross platform (currently running on Mac OS X, Vista and XP, with a clone being written for Linux by the Mono developer teams), that works as a better alternative to Flash. It comes with built in support for video, and it's own .NET runtime and libraries.
