This document discusses using Xamarin to build mobile apps with C# that can run on iOS, Android, and Windows. It highlights that with Xamarin.Forms, developers can share more code across platforms, including shared UI code and a shared backend. It lists several .NET namespaces and platform-specific APIs that are available to Xamarin developers. The document promotes using C# and Xamarin to accelerate cross-platform mobile development for over 2.5 billion devices running iOS, Android, Windows, and Windows Phone.