nixpkgs/nixos/doc/manual/configuration/declarative-packages.xml

<section 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-declarative-package-mgmt">

<title>Declarative Package Management</title>

<para>With declarative package management, you specify which packages
you want on your system by setting the option
<option>environment.systemPackages</option>.  For instance, adding the
following line to <filename>configuration.nix</filename> enables the
Mozilla Thunderbird email application:

<programlisting>
environment.systemPackages = [ pkgs.thunderbird ];
</programlisting>

The effect of this specification is that the Thunderbird package from
Nixpkgs will be built or downloaded as part of the system when you run
<command>nixos-rebuild switch</command>.</para>

<para>You can get a list of the available packages as follows:
<screen>
$ nix-env -qaP '*' --description
nixos.firefox   firefox-23.0   Mozilla Firefox - the browser, reloaded
<replaceable>...</replaceable>
</screen>

The first column in the output is the <emphasis>attribute
name</emphasis>, such as
<literal>nixos.thunderbird</literal>. (The
<literal>nixos</literal> prefix allows distinguishing between
different channels that you might have.)</para>

<para>To “uninstall” a package, simply remove it from
<option>environment.systemPackages</option> and run
<command>nixos-rebuild switch</command>.</para>

<xi:include href="customizing-packages.xml" />
<xi:include href="adding-custom-packages.xml" />

</section>