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.
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.
Drupal Core Modules
TopicA module is a set of PHP, JavaScript, and/or CSS files that extends site features and adds functionality. The Drupal core software comes with a set of modules that provide a range of features.