Kubuntu is an official derivation of the Ubuntu Linux desktop operating system providing KDE support. It is part of Ubuntu. All packages share the same archives as Ubuntu.
It is even possible to install KDE on Ubuntu to achieve a "Kubuntu". Otherway round Gnome can be installed on Kubuntu, so there's no real limitation according to the used desktop environment. Kubuntu comes with a preinstalled and preconfigured KDE and without Gnome.
From the Kubuntu page on the Ubuntu Wiki pages "The Kubuntu project aims to be to KDE what Ubuntu is to GNOME: a great integrated distro with all the great features of Ubuntu, but based on the KDE desktop. Kubuntu is released regularly and predictably; a new release is made with a release of a new KDE Version."
Ubuntu means "towards humanity" in Bemba.
The first Kubuntu release was published on April 8, 2005. It included KDE 3.4 and a selection of the most useful KDE programs not in KDE itself, including amaroK, Kaffeine and Gwenview. Both Live CDs and Install CDs for x86, PowerPC and AMD64 platforms are available. There are also daily builds of the CDs.