Recently Microsoft decided to include Xamarin into Visual Studio, also into the free version. This means that from now you can use the C# language with .NET / Monodevelop framework to develop crossplatform applications with the support not only for Windows, Linux and MacOSX, but also for Android and iOS!
Before this news, you had to pay for Xamarin, but now it's free (with certain conditions, visit xamarin.com). If you didn't want to pay for it, the only way you had to support mobile devices was to rely on existing frameworks, like Qt, Unity and Oxygine, or produce extra code with Android SDK and xCode. The problem is that all these solutions use different kind of languages. Qt and Oxigine are C++, Unity is a 3D engine that uses C# scripts, Android SDK and xCode for iOS are mostly Java oriented. If you wanted to support multiple platforms before, you had to change your habits to adopt a solution (even if you didn't like it) to cover an high range of machines. Now you can continue to develop your project with Visual Studio in C# and then decide to convert part of your project to make a mobile app using the same framework, with a little bit of effort for the platform specific features. If you want to develop an app in short time and share it to the world, Xamarin will make your life easier.