Symfony is a PHP web application framework and set of reusable components. It was first published in 2005 and is inspired by the Spring Framework. The latest version is 4.08 while 3.4 is the current long term support version, which is used by Drupal 8.5. Symfony uses the MVC pattern and includes components like Twig for templating, Doctrine for ORM, and Swiftmailer for email.