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.

Use the Configuration API to access configuration data stored by a module, provide default configuration, create settings forms, and other tasks such as overriding settings for a local environment.

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.