­what is the dividing line between Nix and NixOS?

i’ve seen the definitions of course, but i don’t feel like i fully understand what goes into building a NixOS configuration, as opposed to installing packages with Nix on some other OS. is my NixOS configuration just another derivation like any other that i could potentially write in Nix? does this derivation simply produce a lot of system init files in Nix Store, and then the nixos-rebuild-ng utility is needed on top of that to move those files to appropriate system locations? or is it about...