Usually, extension of Drupal core is called module. But official term for extensions are Drupal contributed modules. Also, Drupal core is consists of modules, but those modules are called Drupal core modules. Drupal developers can code new modules when there is a need for making a new ability like a web shop. Drupal Commerce | Drupal.org is a set of modules for web shop, and it is not part of the core module, it is an extension. There are three types of modules:
