Silverlight is a new web presentation technology that is created to run on a variety of platforms. It enables the creation of rich, visually stunning and interactive experiences that can run everywhere: within browsers and on multiple devices and desktop operating systems (such as the Apple Macintosh). In consistency with WPF (Windows Presentation Foundation), the presentation technology in Microsoft .NET Framework 3.0 (the Windows programming infrastructure), XAML (eXtensible Application Markup Language) is the foundation of the Silverlight presentation capability.

Silverlight is the next step in evolving the potential user-experience richness in which application developers and designers can present to their clients. It does this by allowing designers to express their creativity and save their work in a format that will work directly on the Web. In the past, a designer would design a Web site and a user experience using tools that provide a rich output, but the developer would have to meet the constraints of the Web platform in being able to deliver them.

In the Silverlight model, designers can build their desired user experience and express this as XAML. The XAML can then be incorporated directly by a developer into a Web page using the Silverlight runtime. Thus, the two can work more closely than ever before to provide a rich client user experience.

As XAML is XML, it is text-based, providing a firewall-friendly, easy-to-inspect description of the rich contents. While other technologies-such as Java Applets, ActiveX, and Flash-exist that can be used to deploy richer content than DHTML/CSS/JavaScript, they all send binary content to the browser. This is difficult to audit for security, not to mention difficult to update, as any changes require the entire application to be reinstalled, which is not a user-friendly experience and can lead to stagnation in pages. When SilverLight is used, and a change is needed to the rich content, a new XAML file is generated server-side. The next time the user browses to the page, this XAML is downloaded, and the experience is updated without any reinstallation.

At the heart of Silverlight is the browser-enhancement module that renders XAML and draws the resulting graphics on the browser surface. It is a small download (under 2 MB) that can be installed when the user hits the site containing the Silverlight content. This module exposes the underlying framework of the XAML page to JavaScript developers, so interaction with the content on the page level becomes possible, and thus the developer can, for example, write event handlers, or manipulate the XAML page contents using JavaScript code.

Silverlight provides a retained mode graphics system, similar to WPF and integrates multimedia, graphics, animations and interactivity into a single runtime environment. 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).) Silverlight can also be used to create Windows Sidebar gadgets for Windows Vista. (This, however, will become less true as more search engines (currently only Google) adopt the new Adobe flash player for search engines.

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 file format. Furthermore, the Software license agreement says VC-1 is only licensed for the "personal and non-commercial use of a consumer". Silverlight does not support 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 common dynamic programming languages like Ruby and Python.

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. A third-party free software implementation named Moonlight is under development to bring compatible functionality to GNU/Linux.