Microsoft Silverlight is like Flash but use Microsoft .NET language instead of ActionScript, this help ASP programmers to use Silverlight to create interactive web applications with out studying a new language like Action Script.

Microsoft Silverlight is a browser plugin that allows web applications to be developed with features that characterize a rich internet application: animation, vector graphics and audio-video playback. Silverlight competes with products such as Adobe Flash, Adobe Flex, Adobe Shockwave, Java FX, and Apple QuickTime.

Silverlight was developed under the codename Windows Presentation Foundation/Everywhere (WPF/E). It is compatible with multiple web browser products used on Microsoft Windows and Mac OS X operating systems. Mobile devices, starting with Windows Mobile 6 and Symbian (Series 60) phones, will also be supported.

Silverlight provides a retained mode graphics system, similar to WPF and integrates multimedia, graphics, animations and interactivity into a single runtime. It is being designed to work in concert with XAML and is scriptable with JavaScript. XAML can be used for marking up the vector graphics and animations. Textual content created with Silverlight would be more searchable and indexable than that created with Flash as it is not compiled, but represented as text (XAML).[2] Silverlight can also be used to create Windows Sidebar gadgets for Windows Vista.

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http://www.microsoft.com/silverlight/default.aspx