What is Silverlight and how is different from flash? Why should I install it?

Wikipedia on Silverlight:

Microsoft Silverlight is a web application framework with a scope similar to Adobe Flash. Version 2, released in October 2008, brought additional interactivity features and support for .NET languages and development tools.

So basicly, it is a competitor to Flash that uses .NET languages and is more focused on developing interactive web applications, rather than animations and movies.

Currently, Silverlight is only available for Windows and OS X, but there is an open source implementation being developed by the Mono team entitled Moonlight.

I plan to install Silverlight only when:

• I stumble on Silverlight application which is so great that I will want to go through the hurdle of installing Silverlight
• I stumble on so many Silverlight applications that I can't bear browser nagging me to install Silverlight anymore

So far, I've seen Silverlight content on two random links I clicked, which didn't interest me much anyway, so I'd say that I won't install it anytime soon. :)