Xamarin is a Microsoft-owned company that allows developers to write native Android and iOS apps using C# with a shared codebase. It provides tools like Visual Studio and Xamarin Studio to build, debug, and deploy mobile apps. Xamarin apps can use native UI controls and APIs, while only having to write the business logic once. This simplifies development for multiple platforms. Xamarin also offers services like Test Cloud for testing apps on many real devices simultaneously in the cloud.