Ubuntu's default desktop environment is Gnome Unity. 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.
KDE will start downloading and installing. You can click In Progress if you want to see the percentage of the task that's complete.
Note: this will take significantly longer than other software packages to install, because this actually brings in a lot of packages, not just one. The Kubuntu Plasma Desktop system is what is called a metapackage, which means it's an empty package, which is just a pointer to a whole group of other packages.
You'll see various icons load up.
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.
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.
If you decide you want to remove KDE, it's not enough to go to Ubuntu Software Center and remove the Kubuntu Plasma Desktop system package, as that is just an empty package that points to all the software packages that make up a default Kubuntu installation. Removing the pointer package won't remove all the packages it points to.
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