This page is an example of what Silverlight can do for you:
Silverlight Shock game on nokola.com
As Jofm5 says, it's Microsoft's attempt at doing what Adobe Flash/Shockwave does. In some ways, it does it better in as much as Silverlight applications tend to be smaller (in terms of bytes used) and so download faster.
By all means install the Silverlight runtime - it won't do any harm. And I'm pretty sure that, once you install the runtime, it stops the annoying modal pop-ups which SD mentioned.
Happy zapping
Ken