I don't think it is possible to fully answer this question, as it may be too broad or opinion based, but the Debian Wiki on Children Software distributions gives a pretty good overview of Ubuntu's goals:
Ubuntu is a Linux distribution that starts with the breadth of Debian
and adds regular releases (every six months), a clear focus on the
user and usability (it should Just Work, TM) and a commitment to
security updates with nine months of support for every release. Ubuntu
ships with the latest Unity and KDE releases as well as a selection of
server and desktop software that makes for a comfortable desktop
experience off a single installation CD.
The Debian Manifesto from 1994 gives you a glimpse of what Debian as a project tries to accomplish:
A.1 What is Debian Linux?
Debian Linux is a brand-new kind of Linux distribution. Rather than
being developed by one isolated individual or group, as other
distributions of Linux have been developed in the past, Debian is
being developed openly in the spirit of Linux and GNU. The primary
purpose of the Debian project is to finally create a distribution that
lives up to the Linux name. Debian is being carefully and
conscientiously put together and will be maintained and supported with
It is also an attempt to create a non-commercial distribution that
will be able to effectively compete in the commercial market. It will
eventually be distributed by The Free Software Foundation on CD-ROM,
and The Debian Linux Association will offer the distribution on floppy
disk and tape along with printed manuals, technical support and other
end-user essentials. All of the above will be available at little more
than cost, and the excess will be put toward further development of
free software for all users. Such distribution is essential to the
success of the Linux operating system in the commercial market, and it
must be done by organizations in a position to successfully advance
and advocate free software without the pressure of profits or returns.
So basically Ubuntu sets itself up as an easy to use distribution with solid user interfaces and sane defaults. Debian focuses on solid support and spreading the influence of Linux in professional and end-user settings.