Ubuntu uses Unity which is a shell that works ontop of gnome3. Gnome shell is different again and is the shell which was intended to be run ontop of gnome3, but it doesn't come default with Ubuntu. Gnome3 with out a shell is just a black screen. Kde is a complete different desktop environment but it also uses a shell, it's called plasma-desktop; and once again you can have kde without plasma-desktop but it will just be a black screen. This may be confusing, but as Tron pointed out, the desktop environment is made up of many things, and these shells are just one part.
Xfce another desktop environment uses the traditional method, and doesn't have a shell. Xfce's different 'parts' are all seperate. I'll give an example to help define that better. kde's shell, plasma-desktop consist of things such as, the way the background is handled when you click it ect; the panels; the menus and icons, ect. this is all one great big application called plasma-desktop; the kde shell (although not often described as a shell). Xfce's background, and panels ect are all seperate though. xfdesktop controls the background and icons, xfce4-panel controls the panels. Ok so hopefully i didn't confuse you and have explained that clearly. I don't use xfce, but do like there design desision better than kdes and gnomes as it allows for more flexibility.
Now specifically to your question.
Most distros take the sensible option and only give them selves one name, and when you visit there websight to download there iso, they will often list about four isos, each clearly labeled which desktop environment on each one. Nothing different on each iso except for the desktop environemnt.
Ubuntu does not take the above aproach and instead gives each iso a differnet but similar name, but besides that, it's no different.
Ubuntu uses Unity as its desktop environment, which is a customized gnome3. Kubuntu uses kde4. In the past, Ubuntus kde was not very well implimented. It was very buggy and didn't seem to get the attention other distributions gave to kde. it's picked up on that slack since then though, and offer nice upgrades to kde if you so desire; but it's still Ubuntu underneith. If you like Ubuntu, and also kde, then why not give kubuntu a go. If you already have Ubuntu installed, you can install kde with the following command, if my memery serves me right.
sudo apt-get install kubuntu-desktop
This will not overwrite Unity, when you login you have the option of which one to use.
Hope that helps