Please note that this article is badly out of date since the release of .NET Core and Visual Studio for Mac. An updated article is in the planning stages.
Mono is the open source .NET runtime for Windows, Linux, and OS X. It consists of the Mono runtime environment, libraries, and C# and F# compilers. Recently Mono has gained extra popularity due to Microsoft’s purchase of Xamarin, the makers of a cross-platform toolkit of the same name.