Silverlight is effectively Microsoft's alternative to Flash. But if you say that in front of any Silverlight developer then you'll probably receive a punch in the face. I believe it's much more powerful than Flash, as in being able to do so much more. Silverlight supports such a wide range of technologies such as the .NET framework with the ability to be developed in VB, Visual C#, XAML/XML and JavaScript. I think one of the things they're trying to push with it is online HD video streaming (obviously with a decent Internet connection etc.).
Another thing it can support is Deep Zoom... I don't know if you've heard of it. Basically, it's an effective method of viewing high quality media on the web that also caters for slow Internet speeds (imagine it being just like how Google Maps works; it only downloads the higher quality version of the image when you zoom into that specific area). Deep Zoom is pretty smart to be fair and its developer tools are free.
Also, Windows Phone 7 mobiles supports Silverlight applications that are developed by anyone. So if you have a WP7 phone, then you'll be able to knock up a Silverlight application in Visual Studio 2010 (requires the Silverlight 4 runtime installation and the appropriate SDK) and whack it on. If you don't have an actual WP7 phone to hand then you can use the WP7 emulator that Microsoft provide for free. I've been using the emulator recently... it's pretty cool.
Flash. I'm not a fan of Flash, not really too sure why. I think back in the day (when I was starting to get into website development (about 2001 properly)), my perception of Flash wasn't a great one. Pretty much every time I seen it on the web it was used pure tackily and just ruined the overall deliverable, pure scruffy and that. Since then I've never really looked into Flash and ActionScript, but I'm sure I'd be able to pick it up. Maybe I was a little harsh in my post above lol.
Anyway, here's a few links (check out the Hard Rock Café links for a pretty good example of Deep Zoom):