Ubuntu is based on Debian,
I tend to think of Debian as being the bleeding edge of the Debian based distros. Ubuntu takes that and rounds it into a more user friendly product for end-users. I don't typically think of Ubuntu as a server OS, but I suppose it can be.
Compare this to the RedHat side of things, Fedora is kind of the test bed, bleeding edge distro. Then that gets rounded into form by RedHat Enterprise and CentOS. I haven't checked Fedora is several years, but I was thinking at one time it's life cycle was only about 6 months, so it was never a solution for a server OS. But it allowed Redhat to test out features and changes and get end-user feedback before pushing those changes out to RHEL.
In the Debian world, the Debian distro is more like the Fedora distro and Ubuntu is more like the RHEL distro. It's a loose comparison, but it generally works.