This is by no means all encompassing but...

Start with a family, Debian, Redhat, Arch, Slackware, etc. Then look at their most popular forks. The biggest differences in family is whether it is a rolling release that evolves from update to update, point releases that do major version releases, and hybrids. Then you get into philosophy, like a system that ships with or tolerates non-free software (free as in ideas-open source, not free as in free beer) for the more pragmatic, truly open source (usually requires a degree of skill to have a usable system), cutting edge of tech (usually rolling releases), stability (tested thoroughly, high reliability), and hybrids.

Then there is use case. The overwhelming majority of Linux Kernel based operating systems are used in servers, network equipment, IoT, embedded systems, and mobile. Android uses the Linux Kernel. For the desktop and workstation you have to consider desktop environment. Some distros integrate one particular DE and features better than others for a task.

If all you want to do is get online and stream HD video, then Ubuntu, Linux Mint, PopOS, or MX Linux is for you. If you are passionate about really learning about your system and controlling what it does and how it does it, Gentoo or Slackware may be for you. If cutting edge and understanding is important, Arch. If cutting edge but not so much work as Arch, Manjaro. Redhat and Suse are aimed at enterprise. (Ubuntu server is, but is also aimed at the needs of smaller entities.) Fedora is a testing ground for Redhat, but is popular with enterprise users.

There are also all of the niche offerings for lightweight, security, penetration testing, network evaluation, to name a few.