[JamesMontemagno]

The approach with Xamarin is that you are able to build out a shared C#/.NET business logic layer. All of your platform independent code such as Models, ViewModels, Database code, Restful Service calls, etc. You get to use the power of C# and .NET to build this out. Then you build out user interfaces for iOS, Android, and Windows and tie this logic all together. Xamarin apps are native so you follow the same paradigms for building the UIs out such as Android XML and iOS Storyboard or XIB files, but you can build these out in the Xamarin Designers for both iOS and Android inside of Visual Studio or Xamarin Studio.

On the code behind side of iOS/Android (UIViewController or Activity) you write all of this in C# with C# features like LINQ, events, delegates, etc. and you have access to 100% of the iOS and Android APIs of the platform as well.

Now we do have the Xamarin.Forms library which adds on top of this to build on a shared UI layer for iOS, Android, and Windows Phone as well. However it is not for every app and we have guidance at www.xamarin.com/forms. Hopefully this is a nice overview of the platform.