Adobe's Flash is a tool that allows Web site developers to create Web-based animations. Microsoft announced last year in April that they were planning to create something to compete with it, named Silverlight.
Microsoft Silverlight hasn't really taken off - I really hadn't heard much about it until recently - Microsoft has made the Web browser plug in available through Windows Update, and have announced that they are planning to redesign their entire Web site using Silverlight.
Story about Microsoft planning revision of their Web site in Silverlight
Wikipedia article on Silverlight