Nix channels are basically "where the downloaded packages come from". By default, on NixOS, packages are downloaded from the nixos channel (Is that really a surprise?)

In general, when it comes to channels, there are 3¹ channels that are available:

There are two major methods of using different channels in your configuration.nix file. Which one you choose is entirely up to you, each have their own perks and drawbacks.

Warning

There are multiple different "defined" NixOS channels out there. Some are named nixos, whereas others are named nixpkgs. Do not use nixpkgs channels on NixOS. The nixos channels have different tests to the nixpkgs channels and are designed for the operating system as a whole, unlike the nixpkgs channels.

The nix-channel command is designed to manage multiple nix channels on your system. It's easy to update channels to the latest versions and add or remove channels.

• To list all channels that are available in your system, use the --list flag:

  $ sudo nix-channel --list
  

• To add a new channel (for example nixos-unstable), use the --add flag, followed by the URL of the channel and a name to identify it by. The list of channel URLs can be found at https://nixos.org/channels/.

  $ sudo nix-channel --add https://nixos.org/channels/nixos-unstable unstable
  

• To update a channel, use the --update flag, followed by the channel name that you declared when you added the channel:

  $ sudo nix-channel --update unstable
  

Once you've added your desired channels, you can then add them to your configuration.nix file. This can be done using the Nix let expression, as followed:

{ config, pkgs, ... }:

let
    unstable = import <unstable> {};
in {
    # The rest of your configuration here

    environment.systemPackages = with pkgs; [
        # To use unstable packages, precede package names with unstable.
        # as defined in the "let ... in" section above
        unstable.hello
    ];

};

This method gives you much more control over what specific version of Nixpkgs you want to use on your system, however it means that updating channels (if you choose to do so) will also have to be done manually.

The only difference is that you require the following code structure:

import (builtins.fetchGit {
    name = "NAME";
    url = https://github.com/nixos/nixpkgs/;
    rev = "REV";
}) {};

The NAME is some sort of descriptive name for your own benefit, and the REV refers to the Git commit hash of your choosing (This is pretty easy to find on GitHub, it's shown as a 7-character string on the Nixpkgs repository's commits - clicking on the clipboard icon next to it copies the full hash to your clipboard)

An example of using this would be as follows:

{ config, pkgs, ... }:

let
    myNixpkgs = import (builtins.fetchGit {
        name = "nixos-unstable-2018-09-12";
        url = https://github.com/nixos/nixpkgs/;
        rev = "ca2ba44cab47767c8127d1c8633e2b581644eb8f";
    }) {};
in {
    # The rest of your configuration here

    environment.systemPackages = with pkgs; [
        # Exactly the same as the above example
        myNixpkgs.hello
    ];
};

Alternatively, you can easily change all packages declared in the system packages to use your chosen git revision by changing the packages that are brought into scope after the with keyword:

environment.systemPackages = with myNixpkgs; [
    hello
];

The NixOS channel update website provides insights on when the various Nix channels were last updated, including the link to the latest commit that was included for that update. It's the quickest and easiest way for you to browse through the repository to find whether a specific package has been updated or not.