Ubuntu is Based on Debian. The Ubuntu developers add more software to a default installation to make it more novice-friendly.
Debian is a distribution that has been around since the mid-90s, so it's a base for quite a few other distros.
See the DistroWatch page, here. Or the Debian homepage, here.
*EDIT*
Keep in mind, the initial installation of Debian can require a little work in the command line to get it setup. But once you're finished, it's a system that will remain stable and require little to no other configuration.

