What is Silverlight?
Silverlight is a cross-browser, cross-platform plug-in for delivering the next generation of Microsoft .NET-based media experiences and rich interactive applications for the Web.
What Wikipedia says:
Microsoft Silverlight is a browser plugin that allows web applications to be developed with features like animation, vector graphics, and audio-video playback - features that characterize a rich internet application. Silverlight competes with products such as Adobe Flash, Adobe Shockwave, Java FX, and Apple QuickTime. Version 2.0 of Silverlight (currently under development) will provide additional capabilities, including advanced interactivity features. Significantly, version 2.0 allows developers to use .NET languages and development tools when authoring Silverlight applications.
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