NeXT I believe.. I'll fact check in a bit.
Update - don't know. I know a lot of OS X came from NeXT and their OS. But a GIS doesn't show much similarity between it and OS X aside from a pseudo-dock. The menu bar at the top that has been in all OS X versions originated with the original Macintosh OS though.
I also know that things like the dropdown folders originated in Mac OS 7 or perhaps OS 8 (I used Macs between System 6 and Mac OS 8.5, then again between OS X 10.3 and now).
Extra Update - Here you go, this is a pretty good page with the history -
http://appleinsider.com/articles/07/10/08/road_to_mac_os_x_leopard_finder_10_5
So, what you heard is partially true. But OS X's GUI is mainly just an evolution of the original Macintosh OS (which they called System whatever until Mac OS 8 - ie system 6, system 7).
There is a whole mess with Xerox, but I'm going to stay out of that here, its been hashed out and rehashed all over the web with probably every potential argument possible made for the many sides involved. Google will be your friend for that.