- GNOME is the desktop used in Ubuntu.
- KDE is the desktop used in Kubuntu.
- Xfce is the desktop used in Xubuntu.
To switch between the desktops, log out. When you log back in, click the "session" button and select which window manager you want to use. Easy as pie.
To install, you can use Synaptic Package manager, or apt-get (on the command line). However, these methods don't keep track of dependencies, so it's recommended you use aptitude (on the command line) instead:
sudo aptitude updateLink in title as always, or here.
sudo aptitude install kubuntu-desktop
sudo aptitude install xubuntu-desktop