Microsoft Silverlight is a browser plugin and is sort of similar to Adobe’s Flash Player and Flex Framework taken together
Adobe AIR is cross platform desktop runtime that allows developers to build desktop applications using web technologies, it has no real equivalent in the Microsoft world.
Peter Elst also finds it important to differentiate..
So many people ask me – “How is Silverlight?” or “Is Silverlight a real threat to Flash?”.. well, I never even tried Silverlight. (I did install the plugin, but simply never got to a website that uses Silverlight..)
So the only answer I can give is: when aiming at the web industry (us developers), such a difference in user/client perception must result in different adoption trends, evident: I did not try.. but the logo looks kinda cool.