Mono is a free and open source project led by Xamarin, a subsidiary of Microsoft (formerly by Novell and originally by Ximian) to create an Ecma standard-compliant, .NET Framework-compatible set of tools including, among others, a C# compiler and a Common Language Runtime. The logo of Mono is a stylized monkey's face, mono being Spanish for monkey.
The stated purpose of Mono is not only to be able to run Microsoft .NET applications cross-platform, but also to bring better development tools to Linux developers. Mono can be run on many software systems including Android, most Linux distributions...