Mono

Mono is an open source development platform which is based on .net framework. It provides base to developers to build a cross platform application which results in improving developer's productivity.

components of Mono

1. C# compiler : Mono's c# compiler provides all features for c# 1.0,2.0,3.0,4.0 and 5.0.
2. Mono Runtime :The runtime gives a Just in time (JIT) compiler, a library loader, an Ahead-of-Time compiler (AOT), a threading system, the garbage collector, and interoperability functionality .
3. Base class Library : Basic set of classes compaible with .net rfamework classes.
4. Mono class Library : Many class beyound microsoft base class library.

Mono Features

1. Multi Platform
2. Multi language
3. Microsoft compatible API
4. Open Source, Free software
5. Comprehensive Technology Coverage

Compatibility:

Mono currently support everything in .net 4.5 except WPF,WWF and limited WCF and Asp.net 4.5 async stack

Details of .net framework version supported by Mono .Net 4.5

1. C# 5.0 async support
2. Async Base class library upgrade
3. MVC4- partially supported , no async feature supported
4. Asp.net 4.5 Async pipeline - not supported

.Net 4.0

1. C# 4.0
2. ASP.Net 4.0
3. ASP.Net MVC 1, MVC 2 and MVC3
4. System.Numerics
5. Managed Extensibily Framework - Shared with .NET via MS-PL license
6. Dynamic Language Runtime - Shared with .NET via MS-PL license
7. Client side OData - Shared with .NET via MS-PL license
8. EntityFramework - Available since Mono 2.11.3.
9. Parallel Framework and PLINQ

.Net 3.5

1. C# 3.0
2. System.Core
3. LINQ
4. ASP.Net 3.5
5. ASP.Net MVC
6. LINQ to SQL - Mostly done, but a few features missing

Once you get Mono installed , go ahead with a Hello World program

Copy below code in a file test.cs

using System;

public class HelloWorld
{
    static public void Main ()
    {
        Console.WriteLine ("Hello World");
    }
}

To compile use mcs

mcs test.cs

Compiler will create hello.exe which can be run using

mono test.exe

Output : Hello World

More details please refer to the below reference link :
http://www.mono-project.com/docs/getting-started