Kubuntu is an Ubuntu-derived distribution with the K Desktop Environment (KDE). The Kubuntu CDs are made up of Ubuntu's base plus KDE. You can get exactly the same effect by installing Ubuntu and adding the KDE packages from the Ubuntu archives.
Kubuntu is the result of several months effort to get KDE 3.4 into Ubuntu's main repository and create the first major derived Ubuntu distribution. Kubuntu is not a fork of Ubuntu but an official project of it, sharing the same package archive and infrastructure. It is possible to convert an Ubuntu system to Kubuntu or vice versa.