The main differences between Debian vs Ubuntu is thus:
In technical terms...
Debian has three main different versions.. Debian Stable, Debian Testing, Debian Unstable.
Debian Unstable is the development branch. Updates are continious and can be a handfull time to time.
Debian Testing takes packages from unstable once they meet Debian's standards and don't seem buggy. It's generally easier to live with then Unstable, but there are times (like directly after a stable release) were it can realy suck.
Debian Testing is desgined to form the basis for the next Stable version.
Debian Stable is the current 'production quality' version, which means that if you deploy it you can trust it to be relatively bug free and that it is static.. it won't require any updates beyond major bug fixes or security patches.
Most experianced Debian users will use Testing or Unstable for their personal system and use Debian stable for production servers.
Now Ubuntu...
Ubuntu takes a snapshot of Debian Unstable and then updates or modifies many packages. They update Gnome, update X, change the kernel, and change the Init system. Those are the major changes, but they do some tweaks. They only officially support a fraction of packages from Debian. Most Debian packages they use will end up in the 'universe' or 'multiverse' packages.
Now one thing to keep in mind is the concept of 'Stability'. One meaning of 'stability' with software is a measure of realative 'bugginess'. The More 'buggy' the software is the less 'stable' it is. All software has bugs and all software will crash, so it's a relative way of looking at operating systems.
The second definition of 'Stability' is in terms of 'changing-ness'. If a OS goes through a lot of 'churn' and a lot of changes then it is going to be 'less stable' then a OS that doesn't go through a lot of changes.
Now, of course, both go hand in hand. If you have a OS that changes very often then there is less time for quality control and there is more of a chance of that software being buggy. Usually this is so, but it's not nessicarially always true.
Another thing to keep in mind is that Linux-based operating systems and open source/free software undergoes continious improvements. It's the nature of this type of software that it is never finished. It's just a continious evolution and releases are realy nothing more then a development snapshot that has undergone extra quality assurance to make sure that they aren't very buggy.
The concept of 'software versions' or 'software releases' is a bit of a misnomer. It's always changing.
So Ubuntu is less stable in terms of change, in that it releases very often with less quality assurance then Debian. But Ubuntu also has better hardware support and will have better/more userfriendly software because it is using newer versions of everything that Debian Stable uses. Gnome is getting better as time goes by, the kernel is getting better as time goes by, everything is getting better.
Now that was the more technical stuff. The second part is cultural differences between Debian and Ubuntu.
What Ubuntu has realy been able to do well, that Debian has sucked at, is creating a very aproachable and friendly environment around itself. It's wikis are usefull and provide usefull step-by-step ways of overcoming common problems and dealing with common hardware issues.
Ubuntu provides documentation and steps that are usefull for mortals.
Debian tends to go off into intellectual and political tangents that affect end users in negative ways.
Ubuntu has a 'pretty' distribution that focuses on art work, eye candy, and usability.
Ubuntu provides sane defautls for desktops. They remove a lot of choice from the initial install and setup that Debian provides, but this makes it easier for people to avoid the tedious tweaking and software selection proccess that Debian users go through.
Debian is a bit ackward feeling and takes a lot of effort to get it working very well for a paticular use. But this enables to be very flexible.
So even though Ubuntu will have a tendancy to be a bit more buggy then Debian. It provides a much more usefull initial setup and provides a community that is friendly and makes it easier for people to feel comfortable and learn Linux.
So that makes Ubuntu especially suitable for the 'computer power users'. The sort of people that like mucking around with computers time to time, but are not experts in it. This sort of person is the type that would be most interested in Linux nowadays and is going to be the types that you generally seeing asking questions about 'which linux to use'.
So Ubuntu is a pretty easy choice.
Also if you learn to use Ubuntu well, then Debian won't be very hard to learn either.
Generally speaking most people now would recommend Debian for the server and Ubuntu for the desktop. With Ubuntu vs Debian.