“Pyschocats has written very good blog for Ubuntu beginners to know the difference b/w Ubuntu, Kubuntu, Xubuntu, and Edubuntu”
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.