I am now getting more into customizing my Ubuntu experience and want to understand better what all these different things I keep running into are. What is Gnome3 and Gnome2 in relation to GTK3 or GTK2? Are they related at all? Is Gnome3 another version of Unity? What is Unity? What is compiz?

To make this all VERY basic, the core question is:
How is the Ubuntu GUI built? What are the elements from the bottom-up that make up the desktop environment?

Trying to understand this better so I know what I need/want in order to have my desktop the way I would like it. If this question is better suited for a forum of some sort, please let me know and I will understand completely.

Thanks in advance!