Twig is a flexible, fast, and secure template engine for PHP. It is part of the Symfony framework. Twig is easier tag-based syntax, Twig offers a lot more theming level security. Twig is introduced to Drupal. Drupal 8 theme file extension are html.twig instead of .tpl.php.
Symfony is a web application PHP framework which follows the model-view-controller (MVC) paradigm. Symfony is the leading PHP framework to create Websites and Web Applications. Built on top of the Symfony components. Symfony provides an Architecture, Components and Tools for developers to build complex web applications faster by minimizing repetitive coding tasks.
Symfony has huge community, IRC usually has helpful for Symfony Developers.
These are the following Symfony components included in Drupal 8