Ubuntu's default desktop environment is Gnome. Sometimes people using Ubuntu want to try out KDE as well, though.
Warning: having KDE and Gnome together means you'll have cluttered application menus full of KDE applications and Gnome applications. There may be other minor integration issues, but none should impact functionality, and you can find help for those on the Ubuntu Forums.
Even though these instructions are for KDE, the same principle applies for adding Gnome to Kubuntu or XFCE to Kubuntu or Ubuntu. Basically, you install the desktop environment, log out, and choose the desktop environment.
Go to System > Administration > Synaptic Package Manager
Enter your password when prompted.
Search for kubuntu and then right-click the kubuntu-desktop package and mark it for installation.
When prompted to mark a bunch of packages for installation, confirm the selection by clicking Mark
Click Apply in the main window and you'll get prompted with a smaller dialogue window.
In the dialogue window, you'll be asked if you want to apply certain changes. If you want to see the details, click the little arrows to expand the list of what's to be installed. If you want to proceed, click Apply
Wait for the software packages to download.
You'll then be asked if you want to use gdm or kdm as your display manager. The display manager is basically the login screen. Both GDM and KDM will allow you to choose whether you log into Gnome or KDE. GDM may seem to visually integrate better with Gnome and KDM may seem to visually integrate better with KDE. If you think you'll be using Gnome more often, stick with GDM. If you think you'll be using KDE more often, switch to KDM. Ultimately, it doesn't really matter which display manager you pick, and you can always switch later if you'd like.
When you've figured out which one you want to use, click Forward
Wait for the downloaded software packages to install and then quit Synaptic.
At the login screen, go to Options and select Select Session (I'm using GDM for this screenshot, but in KDM there is also a Session button you click on that looks different).
Under Sessions, select KDE and then proceed with Change Session
Log in with your username and password just as you normally would. GDM will ask if you want to change sessions just this one time or if you want to make the new session the default session. If you're using KDM, you'll just log in directly to the session you've chosen, and it'll stay your default session until you explicitly select a different one.
After that, you should be logged into KDE. And if you want to switch back to Gnome, you just log out and choose the Gnome session instead of KDE.
Note that native Gnome applications will be available in KDE, and native KDE applications will be available in Gnome. Some consider this a convenience. Others consider it an annoyance. That's the way it is, though.
