While playing around with Ubuntu Server, which does not have any kind of desktop environment installed by default, I did "apt-get install startx; startx", which showed me a console in a white box, and whenever I ran a GUI application, I could see a very simple window, with which I couldn't find a way to drag around or do whatever fancy things typically expected to do as a GUI window, but it did still work. I even succeeded running the Firefox browser! The mouse cursor was a black X with white outline. And multitasking didn't seem to be possible.
I didn't install any kind of desktop environment such as KDE, GNOME, Unity, et cetera, and I wonder what the "startx" command is running.