No matter where you go on your first visit to nix whenever looking through a nix expression you'll likely see references to <pkgs> or <mypkgs>.

        pkgs = import <nixpkgs> {};
    pkgs ? import <nixpkgs> #import the nixpkgs set.`

The Nix Packages collection (<nixpkgs>) is a set of derivations (+40,000 and counting) for all sorts of packages with support for linux and MacOS. You can see what is in them here: Search Packages. You could also go to the github repo itself and check it out or browse through it online. It is a true market bazaar of packages. Nix expressions are contributed from the four corners of the globe. Whatever software package you wish to build you will likely find a nix expression that will help you build that package <nixpkgs>, and if one doesn't exist you can write your own.