NixOS is a different approach to package management. Instead of installing packages using a package manager, you edit nix files (written in the nix language). Instead of changing individual config files for the programs and services you install (e.g nginx.conf or postfix.conf, each having a different syntax), you configure them using nix configuration files with a unified syntax.

There is a catch: your system becomes immutable and fully reproducible. Clone the file tree under /etc/nix from one system to another, apply it, and they become identical.

The configuration files are written in a functional language that allows you to customise your system as much as you like.

Read more about it here: https://en.wikipedia.org/wiki/NixOS

Edit: I actually use Nix on Fedora (you can install it on other distros without fully migrating!), and I use home-manager to manage my user environment and dot files. It’s pretty cozy.

Edit 2: and to answer the second question, I use it on my servers because it has comparable stability to Debian-likes while also being super convenient to use. A unified approach to server configuration is a win for me.

NixOS is a declarative OS. Instead of installing a software, you specify it in a config (.nix file) and rebuild. Same goes mounts, services etc.

I use it because it is great to be able to revert if I do something stupid.