Mono is a software platform designed to allow developers to easily create 
cross platform applications.  It is an open source implementation of 
Microsoft's .Net Framework based on the ECMA standards for C# and the Common 
Language Runtime.  We feel that by embracing a successful, standardized 
software platform, we can lower the barriers to producing great applications 
for Linux.


Notes from https://github.com/mono/mono:
 - Mono build relies on a working Mono C# compiler to compile itself
   (also known as bootstrapping).

 - If you don't have a working Mono installation, the build process uses
   the 'monolite' distribution, which contains just enough to run the 'mcs'
   compiler. This is considered a "slightly more risky approach" to build Mono.