The Drupal 8 Entity Construction Kit (ECK) Module allows you to use the Drupal admin interface to build custom entity types. Sometimes you need additional data structures in your Drupal site and it doesn’t make sense to create it as a content type, this is where the ECK module comes in. Using it, you can create your custom entity data structures in Drupal 8 and add fields to them, similarly to how you build out content types.