Nick White wrote on Windows Vista Team Blog :
Yesterday we officially unveiled the name for what was previously known as the extremely catchy "WPF/e" (Windows Presentation Foundation Everywhere) at the 2007 National Association of Broadcasters Conference. WPF/e is now Microsoft Silverlight.
OK, so what? Just what is Microsoft Silverlight? It's a new cross-browser, cross-platform plug-in that allows users a new and richer multimedia experience when using a Web browser (for example, it supports 720p HD media playback). Web developers targeting IE, Firefox and even Apple's Safari browser can all make use of Silverlight for a consistently amazing experience. It's also a client- and server-agnostic platform; this means that the runtimes are the same on any platform, allowing you to deliver the Silverlight experience from an Apache web server, if you like -- without requiring any MS software on the server. How's that for interoperability?