This manual tells you how to write packages for the Nix Packages collection (Nixpkgs). Thus it’s for packagers and developers who want to add packages to Nixpkgs. End users are kindly referred to the Nix manual.
This manual does not describe the syntax and semantics of the Nix expression language, which are given in the Nix manual in the chapter on writing Nix expressions. It only describes the facilities provided by Nixpkgs to make writing packages easier, such as the standard build environment (stdenv).