Arguably it's for the developer, but Arturot's current article in his "31 Weeks of Windows Phone Metro Design" looks at the definitions of three elements of Metro UI - the panorama, the pivot, and the page - and the correct way to use each of them.

On the Panorama screen:

They display only the top or featured content for users. They expose a sneak peak. Not all the content - just those top headers and articles. It’s just a sample of the goodness of your app - not your whole app! Panoramas display a summary of the functions and content available in your app. Literally think of them as magazine covers. To design Panoramas, think of them as full spreads of content, not as single Pages but as a whole large spread. It’s fun to design. Want to get inspired to design Panoramas? Explore some popular and well design magazine covers.

via @Arturot

If you're looking to design your application to fit in with the Metro UI style, this is a page that screams "bookmark me!" - and it's going to be a great reference for others who talk about Metro.