It is an evolution.
It is made up of a few things: MonoDevelop (the IDE core, the same core that powers "Xamarin Studio"), the Xamarin extensions, the VS branding and new code that has not shipped in MonoDevelop/Xamarin Studio before.
Our web site goes into some of the details of what is new that is not currently present in Xamarin Studio:
That said, there are various streams of work happening right now that are coming together. Some open source (those will go on to MonoDevelop), some proprietary (those will only be available in Visual Studio Mac).
We will share with the world more details about those features as they start to land. It is taking all my energy to not share them here just yet.