Silverlight is basically Microsoft's response to Flash, even thought it initially only was a tool for streaming video. There are browser plug-ins available for Windows, Linux(limited compatibility) and OS X. Mobile phone plug-ins are also in development.

The user interface is defined in an XAML file and the logic is coded using a subset of the .NET Framework like C# or VB. The different files are then packed into a .xap file. The XAML file is accessible by search-engines, and that is one advantage Silverlight has over Flash, especially if you want to do create a entire site with Silverlight or Flash.