Metro is a typography-based design language (art style). The design language is inspired by the simplicity of road signs and visual symbols. Like the art on road signs, Metro relies on flat shaps, flat icons, color, and contrast to make icons and UI as simple and clear as possible. Metro was launched with Windows Phone 7, but similar Metro principles (including the Segoe typography) started in Microsoft Encarta 95, MSN 2.0, Windows Media Center, and Zune.
Metro was then created as an official design language for Windows Phone, the Xbox 360 dashboard update, and Windows 8. A specially-made version of Microsoft's Segoe font, Segoe WP, is used for the text. Microsoft also added the Metro design principles to other products and services in order to create a unified and distinctive look across its consumer portfolio, including Xbox 360, Windows Live, TechNet, MSDN, Microsoft.com, and the Microsoft YouTube channel.
Here is the introduction video to the Metro design on Windows 8:
Richard Mueller edited Revision 9. Comment: Fixed url of a link, added tag