XAML—the eXtensible Application Markup Language—allows developers to define user interfaces in Xamarin.Forms applications using markup rather than code. XAML is never required in a Xamarin.Forms program but it is often more succinct than equivalent code, more visually coherent, and potentially toolable. XAML is particularly well suited for use with the popular MVVM (Model-View-ViewModel) application architecture: XAML defines the View that is linked to ViewModel code through XAML-based data bindings.
These samples relate to the XAML for Xamarin.Forms documentation which are found here.
If you open the solution in Xamarin Studio, it will not be able to load the Windows Phone project; and if you open the solution in Xamarin Studio under Windows, it will not be able to load the iOS project either.
Charles Petzold