Symfony is a set of reusable PHP components, and a framework for building PHP applications. Drupal makes use of various Symfony components.

Performance and scalability determine how fast your application can serve a page and to how many users at a time.

PHP namespaces provide a way in which to group related classes, interfaces, functions and constants. Drupal uses PSR-4 namespaces to autoload the correct PHP class from a file, accommodating variations in site structures.

Web services allow two or more applications to share data and instructions with one another across the Internet.

The Migrate API can be used to import content from an older Drupal site (Drupal 7 or older), or any other data source, into the current version of Drupal.

Caching in Drupal is controlled by Drupal's Cache API which is used to store any type of data on a permanent or temporary basis. Some types of data tend to take a long time to compute, but utilizing the Cache API in your module can help your site load data more quickly.

Events are one of the ways that module developers can alter or extend Drupal without modifying existing code.

Controllers are the portion of a Drupal module that handle responding to an incoming request with the appropriate response data.

Composer is the preferred dependency management solution for PHP, is used by Drupal core, and is becoming increasingly popular for managing the modules and themes used for a Drupal project.

In Drupal’s core library there are a number of utility functions and classes that, as a module developer, you will find make your task easier or less tedious.

Drupal's Routing API maps the URL of an incoming request to the code responsible for generating the content that is rendered in response.