Ubuntu, Kubuntu, Xubuntu, and Edubuntu are all the same Linux distro using the same base, the same software repositories, and the same release cycle. They just have different artwork, different user interfaces (in most cases), and different default programs installed.
Ubuntu uses a user interface (or desktop environment) called Gnome. Gnome is focused on simplicity and usability. Ubuntu includes a bunch of Gnome-native applications such as Rhythmbox (music player), Sound Juicer (CD player and ripper), Evolution (email client and calendar), and Gedit (text editor). You can find the full list of software packages in ubuntu-desktop here.
Kubuntu uses the K Desktop Environment (also known as KDE). KDE is focused on including a lot of point-and-click configuration options immediately available to end users. Kubuntu includes a bunch of KDE-native applications such as AmaroK (music player), K3B (CD burning), Konqueror (web browser and file manager), and Kopete (instant messenger). You can find the full list of software packages in kubuntu-desktop here.
Xubuntu uses the Xfce desktop environment, which is a lighter one than Gnome or KDE. In terms of its design principles, it has a bit of a balance—presenting in some ways more point-and-click configuration options than Gnome but also retaining some of the simplicity of Gnome. Its main appeal is its speed, though, and it’s ideal for systems with 128 to 256 MB of RAM. Both Ubuntu and Kubuntu can run on 256 MB of RAM, but they’re more ideal for 512 MB of RAM or more. Xfce includes Thunar (file manager), Thunderbird (email client), and Mousepad (text editor). You can find a full list of software packages in xubuntu-desktop here.
Edubuntu uses the Gnome desktop environment but has a different set of default applications from Ubuntu. Its focus is on educational tools. It includes GPaint (an easy to use paint program), Atomix (a puzzle game for building molecules out of isolated atoms), and Xaos (a real-time interactive fractal zoomer). You can find a full list of software packages in edubuntu-desktop here.
Mixing and matching *buntus is possible and often encouraged by the community. If you choose Ubuntu, you are not stuck with Ubuntu. If you choose Kubuntu, you are not stuck with Kubuntu. You can use Gnome-native applications in KDE and vice versa. You can use Gnome- and KDE-native applications in Xfce. You can install education-related programs in any desktop environment. You can install kubuntu-desktop on top of Ubuntu and choose which one you want to log into at the login screen. All four versions of Ubuntu (Ubuntu, Kubuntu, Xubuntu, and Edubuntu) share the same software repositories and available applications. You are not locked into using one version just because it is the first version you installed. From now on, I’ll be referring to all or any of the above versions as simply Ubuntu.