It's a linux distribution that's based on Ubuntu but replaces Unity and some other applications. Some of the replacements are developed by the Elementary OS team themselves.

https://en.wikipedia.org/wiki/Elementary_os

It looks like it's a Darwin/GTK3 open source OS and desktop environment. Previous release "Jupiter" of 2011 was based on Linux. Has its own development library, "Granite". I'm surprised I haven't heard of it before, it does look interesting.

About the move from Linux to Darwin: http://elementaryos.org/journal/changes-coming-elementary-os

elementary is what Ubuntu should have been. Basically, they've built a new desktop environment from the ground up with focus on design and usability. This the first release of the DE (first release was based on Gnome), so it's still a bit immature, but there's already a big following.