Pantheon Shell is a modular, DE-independent desktop shell.
It consists of:
Unlike GNOME Shell or Unity, it's not monolithic "all-or-none" kind of thing. You can use some components from it while not using the others, or replace some of them with apps that suit you better.
Our philosophy of providing sane defaults and avoiding configuration applies here too. For example, to change the look of Wingpanel one has to edit CSS file - not a good thing to do for an average Joe, but provides great flexibility to the distributor.