Matt and Devin's guide to Desktop Operating Systems
Debian is a sort of meta-OS which provides many user and system-focused features to the Linux kernel, FreeBSD, and soon the GNU Hurd microkernel. Debian provides package management, GNU utilities, and more. Debian is the base of many other Linux operating systems including Ubuntu, Kali Linux, TailsOS, and Purism PureOS. Systems like this collaborate with Debian, but have their own distinguishing feature or feature set. This means that even if you don't directly use Debian, you may be using it at the core of your OS.
Ubuntu, as mentioned earlier, is an OS derived from Debian. According to Debian, Ubuntu is popularizing Linux around the world. It is many peoples' first Linux distribution, as it is recommended by many forums and by its users. As an anecdote, I'll include that it was my first Linux distro, recommended to me at the age of 14 by a friend of mine. It is available as a desktop OS and as a server OS now with live kernel patching across the lineup!
Debian is for people who want a rock-solid Linux or FreeBSD OS with a great selection of software packages. Debian is also a great OS to derive for specific purposes like security and penetration testing or for situations when privacy is an absolute necessity.
Ubuntu is an operating system for people who want a Linux distribution that you can use straight out of the box.