Here is the definition on the net...
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 provides additional capabilities, including advanced
interactivity features. Significantly, version 2.0 allows developers to use
.NET languages and
development tools when authoring Silverlight applications.
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. Support for the
Linux operating system by the third-party
Moonlight runtime is under development.