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.