OK, I'll bite. (I assume this is a series question.)
http://www.go-mono.com/c-sharp.html
Question 1: What is Mono exactly?
The Mono Project is an open development initiative sponsored by Ximian that is working to develop an open source, Unix version of the Microsoft .NET development platform. Its objective is to enable Unix developers to build and deploy cross-platform .NET Applications. The project will implement various technologies developed by Microsoft that have now been submitted to the ECMA for standardization.
Question 2: What is the difference between Mono and the .NET Initiative?
The ".NET Initiative" is a somewhat nebulous company-wide effort by Microsoft, one part of which is a cross-platform development framework. Mono is an implementation of the development framework, but not an implementation of anything else related to the .NET Initiative, such as Passport or software-as-a-service.
Mono includes: a compiler for the C# language, a runtime for the Common Language Infrastructure (also referred as the CLR) and a set of class libraries. The runtime can be embedded into your application. It implements of both ADO.NET and ASP.NET.
The Mono runtime engine is considered feature complete.
The Mono C# compiler is considered feature complete at this point and relatively mature. MCS is able to compile itself and many more C# programs (there is a test suite included that you can use). It is routinely used to compile Mono, roughly half a million lines of C# code.
