Microsoft Silverlight is a cross-browser, cross-platform, and cross-device plug-in for delivering the next generation of .NET based media experiences and rich interactive applications for the Web.
By using Expression Studio and Visual Studio, designers and developers can collaborate more effectively using the skills they have today to light up the Web of tomorrow.
There are a bunch of great learning tools for Silverlight at the Silverlight web site. The videos are in several categories including:
Visit the Getting Started with Silverlight to, well, to see how to get started.