Silverlight is a new cross-browser, cross-platform implementation of the .NET Framework for construction and delivering the next generation of media experiences and Rich Interactive Applications (RIA) for the net.

It runs in all popular browsers, including Microsoft Web Explorer, Mozilla Firefox, Apple Safari, Opera. The plugin necessary to run Silverlight is very little in size thus gets installed very quickly.
Microsoft Silver Light is an application framework for running and writing browser plug-in.

The run-time surroundings for Silverlight is available as a plug-in for most web browsers. While early versions of Silverlight focused on stream media, current versions support multimedia, graphics and animation, and give developers support for CLI languages and development tools.

Silverlight is also of the application development platforms for Windows Phone 7.
One thing Silverlight is not though is a competitor to Apollo Adobe’s expertise that lets developers take their online applications & make them in to standalone desktop apps. Apollo developers will be able to take advantage of capabilities that make applications behave properly whether they are online or not.