(1) A programming model for developing and distributing rich Internet applications (RIA) that use graphics, animations or video within the .NET framework. Silverlight was previously known by its code name, Windows Presentation Foundation Everywhere.
(2) A plug-in from Microsoft for delivering media created with Silverlight. The Silverlight plug-in is both cross-browser and cross-platform. Notably, this makes it interoperable with Apple's OS X and both the Safari and Firefox Web browsers, along with Windows and Internet Explorer. Microsoft's decision to create a plug-in for all three major Web browsers is both an acknowledgement that it does not control the browser market and a strategic push drive adoption by both the development community and installation by consumers of online media.
Significantly, Silverlight shifts development from a desktop-centric environment to one based upon the Web browser. This is possible because Silverlight supports Microsoft's Common Language Runtime (CLR), which allows both designers and developers to run the .NET environment within a browser and do so in a relatively lightweight package (4 MB in total for the Silverlight download). This shift can be seen as both a response to Google's focus on software delivered as a service (SaaS) using the browser and an acknowledgement that the future of computing may be on a distributed network (aka the Internet), not individual personal computers. The introduction of Apollo, Adobe's cross-platform environment for Internet content development, and Sun's long-standing network computing strategy based around Java, points to a strategic shift in the software industry as a whole.
Microsoft has also released a free service called "Silverlight Streaming" that developers can use to store up to 4 GB of content for a Silverlight application, addressing concerns about storage. Silverlight Streaming allows users to stream up to 1 million minutes of online video at 700 kbps at near DVD quality. Silverlight Streaming is integrated into the MSN Video advertising network, allowing content creators to monetize video streams and significantly reduce distribution costs.
Silverlight's introduction can be interpreted as a direct challenge to Adobe's Flash application, with some industry pundits going as far as to call it a "Flash-killer." As Flash dominates the market for rich Internet media creation software, Microsoft will have a significant hurdle to clear in convincing both the development and designer communities to adopt the new platform, to say nothing of introducing yet another browser plug-in for users to download, install and keep updated. Microsoft's large development community, dominance of the desktop operating system market and high browser share are expected to be factors in encouraging consumer adoption of Silverlight.