Metro apps are touch-screen-friendly apps written especially for Microsoft’s WinRT programming interfaces.
ARM-based Windows RT devices, such as the Surface tablet, will require Metro apps. Devices running Windows 8 are expected to be able to run both Metro apps and Win32 apps. According to Microsoft developer Jensen Harris, these apps are to be optimized for touchscreen environments and will be more specialized than current desktop applications. Apps can run either in a full-screen mode, or be snapped to the side of a screen. Apps can provide toast notifications on screen or animate their tiles on the Start screen with dynamic content. Apps can use “contracts”; a collection of hooks to provide common functionality that can integrate with other apps, including search and sharing.
The following links are useful to learn the Windows 8 Metro apps :
Windows 8 Metro Style App Development Official Website
MSDN always would be a great place to start.
Getting started developing apps for Windows 8
Windows Metro App Development Phase
START – http://msdn.microsoft.com/en-US/windows/apps/jj679957
EXPLORE – http://msdn.microsoft.com/en-US/windows/apps/jj680875
DESIGN – http://msdn.microsoft.com/en-US/windows/apps/jj680877
DEVELOP – http://msdn.microsoft.com/en-US/windows/apps/jj680887
SELL – http://msdn.microsoft.com/en-US/windows/apps/jj682079
Language Based Tutorials
For C#/VB and XAML – http://go.microsoft.com/fwlink/p/?LinkId=267207
For C++/XAML – http://go.microsoft.com/fwlink/p/?LinkId=267208
For C++ and DirectX – http://go.microsoft.com/fwlink/p/?LinkId=267210
Channel 9 – http://channel9.msdn.com/Windows
CodeProject TV – http://codeproject.tv/search?utf8=%E2%9C%93&search=Windows+8
Microsoft Virtual Academy –
Developing Windows Store Apps with HTML5 Jump Start
Planning and Preparing for Windows 8
Windows 8 UX Design Jump Start
Windows 8 Metro Apps Templates