By contrast, Ubuntu derives its packages from Debian Unstable or Testing. Instead of being organized by testing status, Ubuntu's repositories are organized by other criteria. Main contains software supported by Canonical, Universe software supported by the Ubuntu Community. Restricted contains proprietary drivers, while Multiverse holds software with copyright or legal limitations.
Another major difference is that Debian is much more dedicated to software freedom. By default, it installs only free software, and its installer even goes so far as to offer a kernel free of proprietary firmware. If you want nonfree software, you have add the Contrib and Nonfree sections to each repository.
By contrast, the distinction between free and proprietary is much less clear in Ubuntu. While Debian discourages the use of proprietary software (although allowing users to make their own choice), Ubuntu encourages users to install proprietary software in order to have a computing experience comparable to that on any commercial operating system. You can get the same experience on Debian if you are willing to work, but Debian makes clearer that you do so at the expensive of software freedom.
For users who may become involved with developing, Debian's and Ubuntu's communities may also be a factor in their choice.
Debian is famous -- even infamous -- for discussing everything in great detail. Especially contentious issues can even go to a general vote.
In recent years, Debian appeared to mellow, but the discussion can still sometimes become a free-for-all. In the past, Debian has been described as a hostile environment for women, and, just recently, the debate on replacing init with systemd became so intense that a couple of veteran developers resigned rather endure abuse.
At the same time, Debian is a meritocratic democracy, with all official maintainers voting on the Debian Leader and other issues. Although appointed positions can sometimes be a source of power, in general Debian officials lead more by suggestion and diplomacy than direct command.
Ubuntu contrasts with Debian in that it has a Code of Conduct for community interaction. Until recently, Ubuntu's community manager was Jono Bacon, who literally wrote the book on The Art of Community, and made considerable effort at conflict resolution. In addition, a Community Council and Technical Board is partly elected every year.
However, this democratic appearance is not quite what it appears. Ubuntu's founder, Mark Shuttleworth, sits on both governing boards permanently, and casts the deciding vote. Shuttleworth -- and, at times, his Canonical representatives -- also have veto power over the community, which in the past has caused several user revolts.
Beginner or expert? Platform support? Ease of use or control? Unity or GNOME? Cutting edge or stability? Free or proprietary? Outspoken but democratic, or polite or controlled? As you can see, choosing between Debian and Ubuntu comes down to what is important to you.
Before you choose one distribution over the other, I suggest that you decide where you stand on all these dichotomies. Or perhaps some of these divisions is more important to you than the others.
However, no matter how you decide, you can hardly go too far wrong. For all their differences, Debian and Ubuntu did not become the leading distributions in free software by chance. Their joint dominance suggests that either is a valid choice, so long as you understand your priorities.