What exactly is "Metro"?

http://www.microsoft-watch.com/article2/0,1995,1789314,00.asp

Its a new version of Sharepoint.  Its a new workflow product.  Its a page description language.  Its a floor-wax.  Its a new printer-driver specification.  Its a desert topping.  What the heck is the dang thing?

It sounds like this is a page-description language similar to a PDF.  Well, that's fine.  The problem that I read from the article is that it implies that this will be a feature of Longhorn.  Well, there are already page description languages that run on WinNT4, Win2k, WinXP, Win2k3, and so on.  The MS approach is so often to create something that only runs on their latest OS or on something that is coming out in the future.  To get there, a company has to go through all kinds of trouble to keep up on the latest and greatest.  Yeah, I know about SMS, but that only replaces sneakernet, not the testing that has to go on.  The expense and heartache is enormous, as I write this on a Windows 2000 system.  Since MS has announced that Avalon graphical subsystem will run on Windows XP and Windows 2k3, I hope that "Metro" will run on XP and 2k3 when it ships.