When we think all standards, tools and frameworks for web on the client was already invented and now its time to spread its use, Microsoft comes with a “new” thing: Silverlight.
Silverlight has same functionality of Adobe Flash. You install it on your desktop system and it works as a browser plugin. Silverlight leverages proprietary .NET, thus it is proprietary too.
When it says cross platform, read Windows and Mac only.
Development tools are Microsoft only.
My advise is to stay away from Microsoft Silverlight or any Mono reimplementation as Moonlight (as noted by Roberto Teixeira in comments). It will lock you in into proprietary technologies.
These are some alternatives (name in bold) for such an impressive interactive web functionality:
- JavaFX [home] should be considered as a trully open standards alternative. Altough it is as new as Silverlight, JavaFX leverages all mature Java ecossystem.
- Althought YouTube and other great online video services use Flash to deliver multimedia content, Flash is generally known as evil for web applications. But if you need such a fat client for web, Flash is more cross platform, cross browser and widely used than Silverlight.
- Plain AJAX can also deliver high impact interactiveness. Its capabilities are similar to SVG above.
As happened with Real versus Microsoft media formats, and Java versus .NET, it is expected that when Silverlight gets more popular, the Flash plugin will be removed from default Windows installations (forcing users to explicitly install it), considered as non-strategic (or a competitor) for Microsoft.