The Nix package database is a set of nested records, with the top-level record usually being nixpkgs.
Accessing attributes in nested records is done by chaining names with a dot, just like in many other languages (e.g. nixpkgs.pythonPackages.django).
This full "path" to a package is called the "attribute path".
Every time Nix modifies the system, e.g. by installing or removing a package, it creates a new generation. One can go back to a previous generation of the sytem with the command nix-env --switch-generation $GENERATION.