A <em>module</em> is a set of PHP, JavaScript, and/or CSS files that extends site features and adds functionality. A set of <em>Core modules</em> is distributed as part of the core software download. Additional <em>Contributed modules</em> can be downloaded separately from the <a href="https://www.drupal.org/project/project_module">Download &amp; Extend page on drupal.org</a>.