From the questions in the parent post I wonder if it would be good to have a clear definition of what Nix is, what NixOS is, how they're different, and when to

From the questions in the parent post I wonder if it would be good to have a clear definition of what Nix is, what NixOS is, how they're different, and when to use each? Could just be a short paragraph, but if the site is for people just starting out it might be helpful...