>>Silverlight apps can automatically install the "runtime" within a browser on any supported platform (such as Mac or Linux) -- at least that's the dream. The runtime (or whatever they will call it) is pretty much completely contained in the browser and interacts with the browser rather than the underlying OS. The size of the runtime is small in modern standards, some 2MB (I hear), so it is kind of like installing Adobe reader or Flash plugins. No biggie.
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>Maybe it will kind of like installing activeX?.. :))

The main difference been that Silvelight provides a much strict security model, not allowing your code to go beyond the browser sandbox. For example, you can't directly access the local disk, don't have direct access to the underlying OS, etc. It is quite much as what you can do with JavaScript, from a security standpoint, but with much more features and way better performance.

Regards,