Ubuntu is designed to be a solid foundation for creating further derivatives. In the case of the official derivatives, all the packages used are from the main Ubuntu repositories, so a user from any variant can seamlessly install packages from any other, and even convert completely to a different variant.

Kubuntu

Kubuntu is a user friendly operating system based on KDE, the K Desktop Environment. Kubuntu and Ubuntu are not meant to be seen as distinct projects; Kubuntu is part of the Ubuntu project, and they are both part of one development team that contributes to the whole. Kubuntu is Ubuntu with a different default set-up, the Ubuntu CD contains only GNOME as a desktop environment; the Kubuntu CD contains only KDE, this is the primary difference.

Ubuntu Education Edition (previously Edubuntu)

Ubuntu Education Edition is an additional selection of free and open source software for education to be used with Ubuntu. As part of the award winning Ubuntu project, it creates a system to support classroom and laboratory environments in schools and other educational settings. It is easy to install, free of viruses and allows teachers to focus on learning instead of licenses. 

Xubuntu

Xubuntu is a light-weight operating system intended to run smoothly even on older, less powerful, hardware. It is built on the Ubuntu core, but uses the slim Xfce desktop environment and GTK+ applications wherever possible.