xorg.conf is the settings file for X, which is the window system you're using. There are all kinds of settings in xorg.conf, but among them are resolution, which display drivers to use, etc. It's located in /etc/X11/xorg.conf. I believe each user can have their own xorg.conf in their home directory, but since ubuntu uses gdm, I'm not sure it's loaded. I'd just edit the one in /etc/X11, but make sure you back the working one up.