Xamarin is a tool that can be integrated with visual studio for building mobile application using the language you are comfortable with like C#
With Xamarin you can build an Android application, IOS application using the C# code on your backend.
Xamarin is mainly meant for the development of mobile application using the same language you are working with and using XAML in front end part.
Xamarin offers two commercial products:
1 For iOS
2 For Android
They are both built on top of a version of the .NET Framework based on the published .NET has been around almost as long as the .NET framework itself, and runs on nearly every platform including Linux, Unix, and Mac OS X.
With Xamarin you can build any kind of mobile application using .NET framework and its language