In Windows 8 Metro, applications don't communicate directly with each other. The OS has built-in contracts available out-of-the-box which make it possible that applications interact with each other without knowing each other. This way, one application can expose its own data for others to read. Another contract makes it possible to expose a common search functionality from within an application.
This new way of using these contracts is something that Metro application developers need to learn in order to build a killer application on Windows 8.
Microsoft uploaded a video recording of a webinar entitled "Contracts and Charms in Windows 8," presented by MVP, Gill Cleeren on April 24th, 2012. In this webinar, Cleeren will explain you the available contracts and how to use them to their limits in your application.