The difference is less important than the essential similarity: it is THE SAME OPERATING SYSTEM.
The Wikipedia definition of Ubuntu is true for the entire Ubuntu family: "Ubuntu ( // oo-boon-too) is a computer operating system based on the Debian Linux distribution and distributed as free and open source software, using its own desktop environment."
The DESKTOP ENVIRONMENT represents the difference. Nowadays (August 2012), the Ubuntu 12.04 is the basic OS in all the Ubuntu family, but Ubuntu proper uses "its own desktop environment" (DE). A desktop environment is a graphical user interface (GUI) to use in interacting with your computer's operating system (OS) instead of using the command-line interface.
Ubuntu-proper uses the Gnome desktop environment which now has a user interface called Unity: that is what you look and feel when in contact with Ubuntu . Unity is the new form (shell interface) of Gnome. The difference is in the appearance, but also in the different things you can do and the ways you can do them. This means that the memory use is different.
KDE is the DE now used by Kubuntu. KDE and Gnome/Unity are the fancier and, as a consequence, the heavier on resources. There are some lighter versions of Gnome.
Xfce is a DE made to be used on computers with lower resources while keeping a rather stylish appearance - and is used in Xubuntu.
LXDE - used in Lubuntu - is the lightest of the family - for even older computers theoretically, but keeping a feel that in my opinion is still more stylish than that of Windows XP, not to mention the speed.
Ubuntu-proper = Ubuntu OS + Gnome/Unity DE
Kubuntu = Ubuntu OS + KDE
Xubuntu = Ubuntu OS + Xfce DE
Lubuntu = Ubuntu OS + LXDE
Xubuntu with Xfce tries to give a lot of customization and style out of the box, but with little effort LXDE/Lubuntu will provide the same while being snappy as a cat.
To experiment a bit with them you can install them on top of each other and log off and log in in a different DE just for the fun of it. But they will not work as well as your basic (initial) DE (but that may be - in my case, at least - because of my computer, which works great with Xubuntu-Xfce, and it flies in LXDE, but has problems in KDE and Gnome. If you have Gnome Unity working on your machine, you should not have trouble in trying KDE, Xfce and LXDE.