As a subset of Microsoft's .NET environment, Silverlight broadens the scope for .NET developers, allowing them to run their programs not only on Windows, but on the Mac, and via the Moonlight project, on Linux PCs as well. See Moonlight
Delivered as a plug-in for Internet Explorer, Firefox and other Web browsers, Silverlight supports animation, vector graphics, MP3 and Windows Media formats including the VC-1 video codec. Silverlight programs can be written in Visual Basic, C#, Python and Ruby.
The Windows Interface - Everywhere
Code-named Windows Presentation Foundation/Everywhere (WPF/E), Silverlight is the cross platform version of the Windows Presentation Foundation (WPF) user interface, first deployed in Windows Vista. See VC-1
and Windows Presentation Foundation