Debian is a Linux distribution, which means a “version” or “flavour” or Linux. Many distributions are just variations of other distributions. As you already said for instance Ubuntu is based on Debian, but adds additional stuff on top of it. Devuan is another distribution based on Debian.

Ubuntu is unpopular in some circles because the company behind it made some very unpopular decisions. Like keeping the server software behind their new way of packaging software (“snaps”) closed source.

The init system of a Linux system is the first process the operating system starts, which then starts all the other processes - it initializes the system. It is also often responsible for managing running background processes and other things.

Systemd is one such init system and widely used by most of the biggest Linux distributions. It is sometimes critisized for “doing too much” and taking over more functions of the system than an init system should, but many people think thats what makes it so practical. Devuan was created by people who wanted to use Debian, but not Systemd.

If you do not know about the specifics and want to use Debian, I would recommend going with its default version, as most of the resources you find online will help you with that.