The following is an excerpt from an article at xamarin.com (http://docs.xamarin.com/mac/guides Jan 1, 2013) which answers your question.
What are the differences between MonoMac and Xamarin.Mac?
Xamarin.Mac is built on the open source MonoMac project. Xamarin.Mac is being developed by the maintainers of MonoMac, though contributions are continuing to flow to MonoMac as well.
Xamarin.Mac has these additional features:
- A commercial license to the Mono Runtime
- Bindings to new frameworks
- Creates self-contained application bundles, without external dependencies on Mono.
These are described in detail in the following sections.
Bindings to New Frameworks
The following are new frameworks and APIs that are included in Xamarin.Mac:
- CoreBluetooth
- GameKit
- New MountainLion AppKit APIs
- SceneKit
- StoreKit
- Commercial License to the Mono Runtime
Xamarin.Mac also comes with commercial support from the Xamarin team, and the Xamarin.Mac product has a published roadmap of forthcoming features.
The Mono runtime license is a commercial license that allows developers to redistribute their Mono-based applications without being bound by the terms of the GNU LGPL v2. This allows you to publish both to the Apple App Store as well as distributing applications that embed the Mono runtime without having to provide source code or object files for end users to relink.
Creating Self-contained Application Bundles
Xamarin.Mac extends MonoMac by allowing developers to ship an application bundle that includes both their software as well as the required pieces of the Mono runtime, without requiring your users to fetch the Mono runtime on their own and install it separately on their systems.
This feature is used for AppStore deployments as well.