Contracts are the centralized communication model introduced in Windows 8. It is hard to imagine a Windows Store application that doesn’t need to use them. They make individual Windows Store applications easier to develop since a lot of the interface is delegated to the Metro ‘Charm bar’. They also allow much better cooperation and easier data transfer between applications

For more details refer http://www.simple-talk.com/dotnet/.net-framework/windows-store-app-contracts-/