Xamarin is a software platform that enables you to create cross platform apps that will work on the top three platforms (Android, IOS, Windows) using the same codebase .  The programming language used by Xamarin is C#.

Xamarin has in a way solved a problem most developers face and that is developing the same  app multiple times for each of the top three platforms : Android , IOS and Windows. Without Xamarin developers will have to use different development tools and programming languages to develop the same app for the top three platforms.
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Topics include:
Installing visual studio on mac
Installing visual studio on Windows
Overview of Visual studio and Xamarin development environment
Creating a cross platform test app
Modifying text
Testing your app in an Android simulator
Testing your app in an IOS Simulator
Modifying XAML code

Xamarin can be used to develop native apps using :
Xamarin.form
Xamarin.IOS
Xamain .Andoid.

Using xamarin.form enables you use the same codebase across the top three platforms.