Xamarin tools are built around the idea that the core of every application tend to be platform independent and so could be written in C# (or F#). This gets compiled into native code on some platforms (iOS). Their approach is to build native apps that use the native UI design tools, so for each platform, you have to re-design its UI to follow the UI paradigms of that particular platform.

The result is still compelling: the UI is native, the underlying code is common on all platforms, you get to use 100% of your platform technical capabilities, the app runs as fast as native apps, you get the power of .Net development tools (even Visual Studio if you want) and all the cool features of C# 5.0 (and F#) to build your apps.

Xamarin, through Mono and its mobile development tools is actually building what Microsoft promised but never really delivered: platform independence for .Net (OS/X, iOS, Windows, WinRT, Android, Linux).