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:

• Core modules are part of Drupal core, and they should not be changed ever by the user;
• Contributed modules are already made extensions which are not part of core, like we mention Drupal Commerce. On Drupal.org, there are more than 46,000 contributed modules. These modules also shouldn’t be changed by the user, just in cases of bugs and deprecated code. In that case, hacking must be presented in a bug report in the module’s project page on Drupal.org. All that modules are free;
• Custom modules are made by users for two purposes: Making completely new functionality for Drupal with extending core and contributed modules via OOP method. Yes, via custom modules, we can extend Core and Contributed modules without changing source modules.

Drupal Core modules

Mainframe basic functions of Drupal website is coded in core. It can be extended via contributed or new customized modules. Drupal core should never be hacked. Just certified experts who work in Drupal.org can make changes and upgrades on core. Sometimes, most used and most common contributed modules can be moved to core. For example, Views contributed module was moved to Drupal core in Drupal 8. That was a contributed module in Drupal 7 but due to widespread use it is now a Core module. From Drupal 8 most backend content displayed pages are made in Views. 

Deprecated code

That is part of some software coded in some programming language which is not useful anymore. Deprecated code may occur because of:

• New version of programming language. So some better commands must be implemented;
• Better code idea. So better code must be changed;

Such part of code must be replaced with newer, better and more secure code. Deprecated code will not be immediately abandoned after its recognition, it will always be used for one period of time, then it will be removed.