First, the good news: although they go by different names, theres really no fundamental difference between the Ubuntu brothers.
Ubuntu uses the GNOME interface, while Kubuntu uses KDE. Specifically, the newest version, Ubuntu 5.10, also known as Breezy Badger, is a Debian-based Linux distribution.
On the GNOME side, Ubuntu uses The GNOME Projects GNOME 2.12, while Kubuntu uses KDE 3.4.3.
In short, you simply choose your interface, and you dont have to worry about the rest.
Kubuntu 5.10, which is the one I worked with the most, can also be upgraded to KDE 3.5.
Package upgrading and management
The method you use to upgrade either one is a bit different. If youre using Ubuntu, youll be using Synaptic. This is a graphical package management program for Debians package management program apt.
On Kubuntu, youll be using Adept, which does the same job.
The name of the game is the same with both: They provide an almost mindlessly simple GUI front end to the apt-get program, so you can easily get both new programs and any and all updates for these programs.
People like to yak about how easy it is to download and install new programs and patches to Windows and its programs. Ha! With either Synaptic or Adept, you can get new programs and upgrade every program on your system with a couple of clicks.
Once the applications are in place, getting to the most commonly used applications is easy. Firefox, Thunderbird, OpenOffice.org, and all your other favorite open-source applications are either already on, or can be quickly added to, the Kubuntu menu.