What is "xamarin.android"

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 (Platform Free Software Implementation .net).

Xamarin develops Xamarin.iOS (Xamarin.Ios), Xamarin.Android (tag:Xamarin.android]) and Xamarin.Mac (for Mac OS X), which enable application development in all Mono-based languages 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.

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