Themes provide the HTML, CSS, JavaScript, and other assets that are responsible for the look and feel of your site.
PHP
TopicPHP: Hypertext Preprocessor (PHP) is a popular interpreted programming language that is especially suited for web development. Drupal core and all contributed modules are written in PHP.
Taxonomy
TopicTaxonomy in Drupal provides a way to classify your site’s content. Taxonomy vocabularies are created with terms within those vocabularies.
Forms (Form API)
TopicDrupal provides a standard, secured method for module developers to add forms to a Drupal website. Learn how to use the Form API to build, validate, and submit a custom form.