When installing packages in NixOS, what is the difference between changing the configuration.nix plus nixos-rebuild-switch versus using nix-shell?

My usual process of installing things on NixOS is changing my configuration.nix , executing nixos-rebuild switch, rebooting, and choosing the new (last) OS generation. I know it is also possible to install packages via nix-shell (but I have never used it). What are the differences between the two options? When one is more appropriate than the other?