https://nixos.org/manual/nix/stable/#chap-writing-nix-expressions
https://nixos.org/manual/nix/stable/#ch-expression-language

The Nix language is used to write expressions that produce derivations.
The nix-build tool is used to build a derivations from an expression. 

https://nixos.org/guides/nix-pills/basics-of-language.html
https://nixos.org/guides/nix-pills/functions-and-imports.html