Twig is a template engine for PHP, which is the language used in Drupal. Twig can be used to design templates in generic PHP applications as well as Drupal themes.

Cascading Style Sheets (CSS) files are added as an asset library through a theme’s libraries configuration file. HTML attributes, including classes and IDs, are commonly added in template files with Twig.

The Render API consists of a standard format for describing data using structured arrays and a process for converting those arrays into the HTML a user sees when interacting with a Drupal site.

A layout can describe how various components are arranged on various levels—from an entire page from the header to the footer, to just the “middle” where the dynamic content goes, to individual components. It can apply to templates for managed content or one-off designs for landing pages.