Ubuntu is a predominantly desktop-oriented Linux distribution, based on Debian GNU/Linux but focused on usability and frequent releases, at the expense of platform diversity, openness.
There are various variants including Kubuntu, Xubuntu, Edubuntu, Gobuntu, Ubuntu Studio, etc...
http://www.ubuntu.com/ - Official website of Ubuntu.
http://www.distrowatch.com/ubuntu - Distrowatch.
The Utnubu Debian project aims to bring Ubuntu changes back to Debian.
Ubuntu for Debian developers
Ubuntu inherits to some extent Debian packages, some of them are just rebuild, some other are possibly heavily modified with respect to Debian. For Debian Developers interesting to know more about the fate of their packages in Ubuntu there is a page in the Ubuntu wiki meant to collect all information useful to Debian Developers: