Unity is a Metacity theme.
Metacity is a compositing window manager used by default in the GNOME desktop environment. The development of Metacity was started by Havoc Pennington and it is released under the GNU General Public License.
Before the introduction of Metacity in GNOME 2.2, GNOME used Enlightenment and then Sawfish as its window manager. Although Metacity is part of the GNOME project and designed to integrate into the GNOME desktop, it does not require GNOME to run, and GNOME can be used with different window managers provided that they support the part of the ICCCM specification that GNOME requires.
Metacity is implemented in GTK+, which makes it themeable and makes it blend in with other GTK+ applications.