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.
Gobuntu is a GNU/Linux operating system, derived from Ubuntu, that endeavors to adhere to the Free Software Foundation's four freedoms and intends to provide a base for other free software platforms to build upon with minimal modification required. It does this by only including open-source non-restricted software. This means there will be no firmware, drivers, applications, or content included in Gobuntu that does not include the full source or whose license does not provide the right to use, study, modify, and redistribute the body of work.
Gobuntu shares the same system requirements as Ubuntu. At present, this means Gobuntu is available for 32-bit and 64-Bit PC architectures and the install requires at least 4 GB of disk space. you can get Gobuntu here
source:http://linuxpoison.blogspot.com/2008/05/135781758014557.html