We're now going to cover how to install a new desktop. Say you're running Ubuntu, and you'd like to be able to use the Kubuntu desktop. Simple - open Terminal and enter the following:
sudo apt-get install kubuntu-desktopApt-get will download the kubuntu-desktop metapackage and install it for you. You'll notice that all of the default applications for Kubuntu will be added to you Ubuntu install, so you'll have the option of using Kontact instead of Evolution for e-mail and so on. You'll also be asked which display manager you want to use, GDM or KDM (either is fine, the only visible effect so far as I can see is that if you use GDM it has the Ubuntu login screen, and for KDM the Kubuntu login screen).
Once that is done, you can log out and return to the login screen. From here, you can change the session type from Gnome to KDE using a pull-down menu, then log back in. You'll now be using the Kubuntu desktop!
You can also install the kubuntu-desktop metapackage in Xubuntu. Similarly, you can install the ubuntu-desktop metapackage in Kubuntu or Xubuntu, and the xubuntu-desktop package in Ubuntu or Kubuntu. Each metapackage includes all the applications you need to create a working install of the desktop, but every single application you've installed will continue to be available in all three desktops.
This is a great way to experiment with the three desktops to see which you prefer. By having all three available, it's easy to find out which one you like the best. Also, although I can't deny it does take a while to download each individual desktop, it's quicker than it would be to download Ubuntu, Kubuntu and Xubuntu and install them each in turn to give them a try before moving on to the next one.