If you install gnome (standard ubuntu install) and then
Then you will have a pretty simular set-up. (as installing kubuntu/xubuntu/etc), and one of the other desktops. There are some minor art-work differences with each such as a boot up logo, but there are not any major noticeable differences, aside from a few installed packages.
sudo apt-get install kubuntu-desktop
sudo apt-get install xubuntu-desktop
Ubuntu/Kubuntu/Xubuntu all draw from the same repositories, so aside from whats on the installation CD there is no difference other than the destop will be gnome kde or xfce (or even enlightenment).
That is the difference, the desktop.