Mono is a project led by Novell (formerly by Ximian) to create an Ecma standard compliant .NET compatible set of tools, including among others a C# compiler and a Common Language Runtime. Mono can be run on Linux, BSD, UNIX, Mac OS X, Solaris and Windows operating systems.
Microsoft has a version of .NET available for FreeBSD, Windows and Mac OS X called the Shared Source CLI (Rotor). Microsoft's shared source license may be insufficient for the needs of the community (it explicitly forbids commercial use). The Mono project has many of the same goals as the Portable.NET project.
The Mono runtime contains a just-in-time compilation (JIT) engine for a number of processors: x86, SPARC, PowerPC, ARM, S390 (in 32-bit and 64-bit mode), and x86_64, IA64 and SPARC for 64-bit modes. The runtime will perform a just-in-time compilation to the machine's native code which is cached as the application runs. It is also possible to precache the native image before execution. For other supported systems not listed, an interpreter performs each byte code one by one without compiling the image to native code. In almost every condition the JIT method will outperform the interpreted method.
Reference:
http://en.wikipedia.org/wiki/Mono_%28software%29
