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 and openness.
There are various variants including Kubuntu, Xubuntu, Lubuntu, Ubuntu Budgie, Ubuntu Studio, etc...
http://www.ubuntu.com/ - Official website of Ubuntu.
https://help.ubuntu.com/ Official Ubuntu Help website
http://ubuntuguide.org - Unofficial help manual, UbuntuGuide
http://www.distrowatch.com/ubuntu - Distrowatch.
Ubuntu for Debian developers
Ubuntu inherits to some extent Debian packages. Some of them are just rebuilt; some others may be heavily modified with respect to Debian. For Debian Developers interested in learning 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: