http://www.mono-project.com/about/index.html

Mono is a comprehensive open source development platform based on the
.NET framework that allows IT and ISV developers to build Linux and
cross-platform applications with unprecedented productivity.
Mono's .NET implementation is based on the ECMA standards for C# and the
Common Language Infrastructure.

.NET and its new language, C#, however, offer the performance and
appearance of native code. With the Common Language Specification for
compilers, code can be mixed freely across languages: Java, C, C#, C++,
Python, and so forth. In addition, .NET uses an intermediate compilation
mechanism that allows cross-platform deployment of binary applications.