XAML is in some ways a logical successor to SGML-based markup languages like HTML and XML. Although of course XAML is an XML dialect it provides a way to bind presentational data (the declarative list of UI elements) with some or all of the code used with them. Now why, you might ask, would Microsoft be so bothered about that? Simply put, Microsoft needs to find new ways to exploit the processing power of the client as well as the server, if it is to continue selling operating systems for the next few decades. If true thin-client computing ever really got started, it would eviscerate the OS market. That’s what Google is aiming for, and it’s exactly the thing that Microsoft fears.XAML allows you to export processing to the client machine in a way that mere script doesn’t. In effect, you can stream the interface of an application, plus a portion (or even all) of its logic, over the wire using a protocol that’s open on most firewalls. You can stream XAML into the browser and get something roughly equivalent to ActiveX controls or Java applets (which exports .NET to the browser as Silverlight has done, something Microsoft has been planning for a while). Silverlight is the first fruit of this particular plan.However, don’t expect XAML-over-HTTP to stay within the browser for long. It’s a perfect way to provide desktop apps without installers, software-as-a-service, all the stuff Microsoft was supposed to be planning years back when people were still talking about Windows.NET - you didn’t think they just gave up on all that stuff, did you?Add to this the bare-metal hypervisor stuff they are developing - ‘Viridian’ for Windows Server 2008 will be a hobbled implementation of this, but there’s more coming, and it will likely be the core architecture of whatever ‘Vienna’ turns out to be - and you can see a model developing of (supposedly) safe, sandboxed, virtualised applications streamed from the net over fast connections, always up to date, running partially on the client and partially on the server, potentially replacing today’s stateless browser-based model of internet application.I doubt the reality will live up to the dream - it never does with Microsoft, any more than it does with anyone else - but that’s the reason they want you to buy into XAML in a nutshell.