Metro was the code name of the design system used in the Windows 8 and Windows Phone 7 user interface. It represented a new design language for Microsoft's mobile phones, tablets and PCs. The company called it Metro because it's modern and clean, fast and in motion but the name did not survive the public release of Windows 8. A company in Dusseldorf, Germany called the Metro Group seemed to have taken exception to the use of the name and there was widespread suggestion it was related to a trademark dispute.
Techopedia explains Metro
The developers of this new what-used-to-be-called-Metro UI were inspired by the eye-catching signs that are commonly found in the King County Metro transit stations, which serve the Seattle area around Microsoft’s headquarters. Metro is designed to be modern and sleek.
This UI design centers around interactive and colorful tiles with big typography meant to draw attention. It can be used in place of the traditional Windows PC desktop in Windows 8, but is the only UI choice in Windows Phone 7.
The design principle is to consolidate common tasks and processes and speed up their usage. This is accomplished by removing unnecessary graphics and relying on dynamic content to populate the UI. This results in an interface that favors large hubs or tiles over small icons and buttons, with a laterally scrolling canvas to accommodate more tiles.