Nix is a package manager which comes in a form of many command line tools. Packages that Nix can build are defined with Nix Expression Language.
The Nix Packages collection (Nixpkgs) is a set of thousands of packages for the Nix package manager and NixOS Linux distribution.
NixOS is a Linux distribution based on Nix package manager.
The Community page shows where the community meets.
The Commercial support page provides a list of consulting companies.