Silverlight may be used to deliver Rich Media Experiences or Rich Internet Applications on the Web. V1 of Silverlight focused on Media, V2 focuses on RIA.
All you need to run Silverlight applications is a Web Browser should it be Internet Explorer, Firefox or Safari, on Windows or Mac. If you are a Linux desktop user, you should turn to Novell who delivers the Moonlight runtime, The technology & architecture issues detailled here also apply for Moonlight.
Silverlight 1 is RTM, while Silverlight 2 is in beta 2 as of July 2008 and will be RTM'd before the end of 2008.