I would say that it is about:
1. Release cycles.
Ubuntu is releasing a new version every 6 months. Only LTS versions get relaesed every 2 years.
Debian is releasing a new version every 2 years.
2. The installer
During install Ubuntu is creating one user which can use sudo.
The whole thing has seven steps: the selection of language, time zone, and keyboard, partitioning, creating a user account, and confirmation of your choices. Thats the reason for the "beginners friendly" tag.
During install Debian is creating one user and is asking for the root password.
The installer does offer quite a lot of options. But the latest graphical installer is similar to the one of Ubuntu (in the number of steps).
3. Packages
The Debian package managment allows you to choose your own level of risk and innovation.
If you want the very latest software, you can use Unstable - yup that means problems.
You can choose Stable for well-tested software supported by constant security updates.
Each Debian repository is further subdivided into main (free software), contrib (free software dependent on other none free software) and non-free (software free for the download, but having a non-free license).
Ubuntu's repositories are organized by support status,
Ubuntu's repositories consist of Main (software supported by Canonical, Ubuntu's commercial arm),
Universe (software supported by the Ubuntu community),
Restricted (proprietary drivers), and
Multiverse (software restricted by copyright or legal issues).
4. Community
The Ubuntu forums are more accessible to newcomers (stupid questions allowed), while Debian forums are more technical and closed (don't be a woman, don't say something like "but with windows ....", and don't ask anything that is part of a man page).
Don't forget that Ubuntu is a branch of Debian. They use the upstream of Debian but are adding additional stuff (Cononical) to it.
Ubuntu itself does have some meta-packages like "dovecot-postfix" which do a great part of the configuration. So it is simple to use and you can change whatever you want.
Something that you could be killed for in Debian - if you would suggest such a package.