Linux prides itself on providing choice, and this includes choice of software. that is why many distributions offer a choice of web browser, email program, music and video players and even the complete desktop when you install them - a whole DVD full of choices. This can be confusing to those taking their first steps with Linux, so Ubuntu took the approach of standardising on one program for each task and keeping the whole installation on a single CD.
This is great if you like their choices, otherwise you are left with the job of downloading and installing the alternatives you want after installation, and ending up with two of everything. So Ubuntu have alternative releases with different default choices. Kubuntu is Ubuntu with the GNOME desktop and various GTK programs removed and replaced with KDE alternatives, and non-specific programs like OpenOffice.org are set up with KDE-like theming so that it all looks like a harmonious whole ago, but a KDE whole.
In most respects Kubuntu is just Ubuntu with KDE instead of GNOME (and no brown themes), so if you like Ubuntu but prefer KDE, this is the obvious choice for you. Some of the system programs, like the Synaptic package manager, have been replaced with KDE/QT alternatives, but everything in Ubuntu and Kubuntu (and Xubuntu too) is in the same repositories, so you can add back any GTK applications you want. Ubuntu and Kubuntu are not mutually exclusive choices, but they do allow you to start with an environment that is closest to your need before you start customising and adding packages. Unlike some of the Ubuntu respins available, Kubuntu is an official Ubuntu release.