Object-oriented PHP utilizes classes and objects to organize code into reusable chunks. This approach helps us organize complex applications, such as Drupal, into modular code called classes that can be reused across the entire system.

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

Plugins are one of the ways that module developers can write code that extends Drupal. The Drupal Plugin API allows a module to provide functionality in an extensible, object-oriented way.

Dependency injection is a design pattern commonly used in object-oriented software architectures in order to support Inversion of Control.

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

Services are objects that encapsulate the code for performing specific tasks in a reusable and decoupled way.

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

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.

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.

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.

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.

Upgrading is the process of moving your site from a previous major version of Drupal to a newer version, for example from Drupal 7 to Drupal 11.

As an open source project, Drupal depends on community contributions in many forms including documentation, code, translation, speaking, organizing events, mentoring others, and even donating money.

Drupal has an amazing community of people who create the software and help each other make the best use of it. This is one of the main benefits of using Drupal — and everyone should take advantage of that.

Hooks allow modules to alter and extend the behavior of Drupal core, or another module. As a Drupal developer, understanding how to implement and invoke hooks is essential.

A site’s main navigation elements, whether they be in the header, footer, or sidebar, are composed of links built using Drupal's menu system. Drupal allows site administrators to build menus while also allowing module developers to add and alter menus in code.

A block is a reusable widget that is placed inside regions (layout containers) of your theme. Blocks can be used by site administrators on the Block layout admin page or provided by a module using the Plugin API.

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.

Development environments provide a sandbox where you can work on your application without affecting the live site.