Xamarin.Android exposes a C#/CIL binding to the Android API and also provides access to the ECMA CIL APIs and various other .NET APIs.
API Reference: The API reference is available at http://androidapi.xamarin.com/.
API Design: The API design for the CIL/Android binding is covered in the Mono for Android API Design document and it describes how we bound the Android APIs and exposed them to the C#/CIL world.
Assemblies: Check our list of bound assemblies for a detailed reference of which APIs from C#/CIL/ECMA are available for use on Mono for Android.