Drupal 7 updating image styles
Drupal's module system is based on the concept of "hooks".
A hook is a PHP function that is named foo_bar(), where "foo" is the name of the module (whose filename is thus foo.module) and "bar" is the name of the hook.
Drupal 8 brings a new migration system into Drupal core, with the goal of making it easier to import data from a variety of sources.
The migrate system is both a framework designed to facilitate writing custom migrations, and an implementation of that framework aimed at Drupal-to-Drupal migrations.
Previous versions of Drupal provided an upgrade mechanism that allowed for in-place version updates, which worked for both major version .
In short, automating what might otherwise be a tedious job of copying and pasting.By the end of this tutorial you should have a better understanding of what the migration system is capable of and know where to find more information about how to use it.Know where to get started with your Drupal upgrade or migration project.When Drupal wishes to allow intervention from modules, it determines which modules implement a hook and calls that hook in all enabled modules that implement it.The available hooks to implement are explained here in the Hooks section of the developer documentation.