First of first, one thing we need to know is that C# and CLR (actually C++/CLI) are all EMCA standards now. It means if you want, you can implement your version C# compiler, CLR on other platform. But you are late now. As a project called Mono has already done this! In one word from a pure user perspective, it allows you to write c#/.Net based application on Mac and Linux and other OS that is not Windows! their MonoTouch application can allow you to write iphone application using C# plus iphone SDK on Mac! What a cool project! You can learn more about visiting their site. Below is a the "what is Mono" page:
What is Mono
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.
The Components
There are several components that make up Mono:
C# Compiler - The C# compiler is feature complete for compiling C# 1.0 and 2.0 (ECMA), and also contains many of the C# 3.0 features.
Mono Runtime - The runtime implements the ECMA Common Language Infrastructure (CLI). The runtime provides a Just-in-Time (JIT) compiler, an Ahead-of-Time compiler (AOT), a library loader, the garbage collector, a threading system and interoperability functionality.
Base Class Library - The Mono platform provides a comprehensive set of classes that provide a solid foundation to build applications on. These classes are compatible with Microsoft's .Net Framework classes.
Mono Class Library - Mono also provides many classes that go above and beyond the Base Class Library provided by Microsoft. These provide additional functionality that are useful, especially in building Linux applications. Some examples are classes for Gtk+, Zip files, LDAP, OpenGL, Cairo, POSIX, etc.
The Benefits
There are many benefits to choosing Mono for application development:
Popularity - Built on the success of .Net, there are millions of developers that have experience building applications in C#. There are also tens of thousands of books, websites, tutorials, and example source code to help with any imaginable problem.
Higher-Level Programming - All Mono languages benefit from many features of the runtime, like automatic memory management, reflection, generics, and threading. These features allow you to concentrate on writing your application instead of writing system infrastructure code.
Base Class Library - Having a comprehensive class library provides thousands of built in classes to increase productivity. Need socket code or a hashtable? There's no need to write your own as it's built into the platform.
Cross Platform - Mono is built to be cross platform. Mono runs on Linux, Microsoft Windows, Mac OS X, BSD, and Sun Solaris, Nintendo Wii, Sony PlayStation 3, Apple iPhone. It also runs on x86, x86-64, IA64, PowerPC, SPARC (32), ARM, Alpha, s390, s390x (32 and 64 bits) and more. Developing your application with Mono allows you to run on nearly any computer in existance (details).
Common Language Runtime (CLR) - The CLR allows you to choose the programming language you like best to work with, and it can interoperate with code written in any other CLR language. For example, you can write a class in C#, inherit from it in VB.Net, and use it in Eiffel. You can choose to write code in Mono in a variety of programming languages.