Rest of the story over at TechShout  

It is a proprietary runtime for browser-based Rich Internet Applications (RIA). Silverlight was codenamed as Windows Presentation Foundation/Everywhere (WPF/E) and will let users create content related to animation, vector graphics, and video playback capabilities. From what we saw, Silverlight seems promising but then, it is a Microsoft product and skeptical users should wait for some time before using it. Silverlight has had its fair size of criticism.