In order to really get a grasp on Drupal, especially how Drupal models data, it's necessary to have at least a familiarity with the Entity API. If you're already familiar with the Entity system in Drupal 7, you may be interested in starting with our What's new in Drupal 8 with the Entity Field API video. If you're new to Drupal with version 8 don't sweat. The tutorials in this series assume you're starting from scratch.
We start off with an overview of the terminology associated with the Entity system. We then look at a high level overview to see how the various components come together to form an API. Our configuration management series already has tutorials on Configuration Entities, but in this series we introduce how to construct your own custom content entity. We will also take time to learn how entities can be created, read, updated and deleted programmatically from elsewhere in our code as well as how to use the Entity query service to identify entities matching particular conditions.
This series also includes tutorials on the Entity systems' other hooks, and more about how you can interact with existing entity implementations.