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Microsoft Silverlight is a programmable web browser plugin that enables features such as animation, vector graphics and audio-video playback that characterizes rich Internet applications. Version 2.0, released in October 2008, brought additional interactivity features and support for .NET languages and development tools.

Silverlight provides a retained mode graphics system similar to Windows Presentation Foundation, and integrates multimedia, graphics, animations and interactivity into a single runtime environment. In Silverlight applications, user interfaces are declared in XAML and programmed using a subset of the .NET Framework. XAML can be used for marking up the vector graphics and animations. Textual content created with Silverlight is searchable and indexable by search engines as it is not compiled, but represented as text (XAML). Silverlight can also be used to create Windows Sidebar gadgets for Windows Vista.

Silverlight supports playback of WMV, WMA and MP3 media content across all supported browsers without requiring Windows Media Player, the Windows Media Player ActiveX control or Windows Media browser plugins. Because Windows Media Video 9 is an implementation of the SMPTE VC-1 standard, Silverlight also supports VC-1 video, though still only in an ASF container format. Furthermore, the Software license agreement says VC-1 is only licensed for the "personal and non-commercial use of a consumer". Silverlight, since version 3, supports the playback of H.264 video. Silverlight makes it possible to dynamically load XML content that can be manipulated through a DOM interface, a technique that is consistent with conventional Ajax techniques. Silverlight exposes a Downloader object which can be used to download content, like scripts, media assets or other data, as may be required by the application. With version 2.0, the programming logic can be written in any .NET language, including some derivatives of common dynamic programming languages like Iron Ruby and Iron Python.

Silverlight 1.0, which was developed under the codename Windows Presentation Foundation/Everywhere (WPF/E), consists of the core presentation framework, which is responsible for UI, interactivity and user input, basic UI controls, graphics and animation, media playback, Digital rights management, and DOM integration.

for more http://en.wikipedia.org/wiki/Silverlight