There are many different Editions of Ubuntu and below is a list of Ubuntu Editions I know. Please let me know if you know any other Ubuntu Editions I did not list below.
Ubuntu: This is the main Ubuntu Edition and it is Gnome based with Unity and it used Gnome based Applications primarily.
Kubuntu: This is a KDE based Ubuntu Edition. It used K Desktop Environment and common KDE applications.
Edubuntu: This Edition is focused on providing a Ubuntu that is entirely Linux Based, freely available and with community support for the Educational sector.
Xubuntu: This edition is based on the XFCE Desktop Environment and focused for less powerful Machines
Fluxbuntu: This edition is based on the Fluxbox Window Manager and their focus is on a wide range on computers and mobile devices.
UbuntuStudio: This edition is focused on providing a platform for all audio, video and graphic enthusiast as well as professional.
LinuxMint: This is a Ubuntu based Distribution which focus is to provide a very elegant and up to date version of Ubuntu and easy to use.
Lubuntu: The focus on this edition is to provide a very light weight linux operating system to preserve resources and energy and is based on LXDE (Linux X11 Desktop Environment)
Mythbuntu: Mythbuntu is an official Ubuntu flavor focused upon setting up a standalone MythTV based PVR system and used XFCE as their Desktop.