Xamarin Android allows you to create apps for Android using C# and Visual Studio / Xamarin Studio on PC/Windows or Mac/OSX
Xamarin iOS allows you to create apps for iOS (iPhone/iPad) using C# and Visual Studio / Xamarin Studio on PC/Windows or Mac/OSX.
If using PC a Mac build host is still required, but cloud services can be used.
Windows 8.1 and Universal Windows Apps can be created with Xamarin by loading the page/view content from a PCL using Xamarin.Forms.
Xamarin is a cross platform framework for developing all device mobile apps using C# and Visual Studio (PC) or Xamarin Studio (Mac)
Xamarin.Forms allows true cross platform all device development by compiling native apps from a shared PCL code base where XAML can be used to design the views.