What is "xamarin"

Xamarin was founded in May 2011 and is established in San Francisco and its engineering office is located in Cambridge, MA.

Xamarin developed a product called Xamarin Studio, which enables the development of native mobile applications for iOS and Android in addition to Mac OS X using c# through the framework based on mono (implementation in Free Software platform .net).

Xamarin develops Xamarin.iOS (Xamarin.Ios), Xamarin.Android (Xamarin.android) and Xamarin.Mac for OS X, which enable application development in all languages based on mono with full access to native Apis. This makes it possible to share code across platforms and create applications for two platforms that have one look and Feel native.

Now Xamarin, after being purchased by Microsoft, has a plugin for Visual Studio 2015 as can be seen in this topic of MSDN

Read more on the subject at Xamarin.iOS documentation, to Xamarin.Android documentation and the Xamarin.Mac documentation.