nixpkgs/nixos/doc/manual/configuration/package-mgmt.xml

<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="sec-package-management">
 <title>Package Management</title>
 <para>
  This section describes how to add additional packages to your system. NixOS has two distinct styles of package management:
  <itemizedlist>
   <listitem>
    <para>
     <emphasis>Declarative</emphasis>, where you declare what packages you want in your <filename>configuration.nix</filename>. Every time you run <command>nixos-rebuild</command>, NixOS will ensure that you get a consistent set of binaries corresponding to your specification.
    </para>
   </listitem>
   <listitem>
    <para>
     <emphasis>Ad hoc</emphasis>, where you install, upgrade and uninstall packages via the <command>nix-env</command> command. This style allows mixing packages from different Nixpkgs versions. It’s the only choice for non-root users.
    </para>
   </listitem>
  </itemizedlist>
 </para>
 <xi:include href="declarative-packages.xml" />
 <xi:include href="ad-hoc-packages.xml" />
</chapter>