From: porsupah	Date: January 25th, 2004 07:32 pm (UTC)		(Link)

Pretty much - X11 is the foundation layer for most Unix UIs, with the notable exception of OS X, which uses Quartz, based on Display PDF (hence the system-wide ability to print to PDFs, as well as printers and faxes). X11's been around for quite a while, hence its high adoption ratio. Gnome and KDE are two popular window managers which sit on top of X11, fwiw. One particularly nice aspect of X11, and OpenStep's Display PostScript, is that it's a client-server system, so the two can be running on different systems entirely transparently; OS X unfortunately lost that capability in the transition from DPS to DPDF. (Reportedly, Adobe wanted an insane amount for licensing DPS again, so Apple rolled their own. A well-integrated implementation of X11 is included with Panther, though, so folks can run X11 programs too, and have them look much like a "native" app)