To the Drupal developers getting started with Symfony, there's a whole new set of vocabulary words we need to learn. Let us check the new terminologies here...
Yaml is not another Markup language.It is used to define .info files and other configuration
Composer is a tool for dependency management in PHP. It allows to declare the dependent libraries a project needs. In Drupal, it does the work formerly handled by the Libraries Module.
PSR-4 is a standard for autoloading classes. Your classes namespace always start with Drupal\your_module\ on disk that looks like: docroot/modules/custom/your_module/src/YourClass.php
Dependency Injection is the preferred method for accessing and using services in Drupal 8 and should be used whenever possible. It declaratively express dependencies in the class definition rather than instantiating in the class itself.
Service Container, also known as Dependency Injection Container, is a PHP object that manages the instantiation of services i.e. objects. Service is any PHP object that performs some sort of global task.
"Routing System" works with the Symfony HTTP Kernel. It can do everything Symfony's can (and more), and both use the same syntax to define routes. A route is a path which is defined for Drupal to return some sort of content on.