Mono (the other white meat) is a technology created by the open source community to provide Microsoft .NET technologies on non-Microsoft platforms like Linux, Mac OSX, and now iPhone.
Read this article for a description of Microsoft.NET technologies:
Microsoft .NET Technologies
Mono is simply the Linux and Mac version of that technology. Mono is not supported by Microsoft in any way. As such, Mono gets updated with new capabilities after .NET is updated with the latest. Once .NET is updated, the Mono development team examines 
what’s new to .NET, then they implement their own code to provide the same capabilities. In general, Mono is about 6 months behind the latest Microsoft .NET technologies.
To download the Mono framework or to become involved in the Mono development team, go to the mono web site.
In addition to Windows, Mac, & Linux, Mono is now available for the iPhone with a product call MonoTouch. Now, you don’t have to learn a proprietary language (objective C) to write iPhone applications. This will make iPhone development MUCH easier.