Fields

The principal way data is modeled by entities is through fields. Entities themselves are essentially just a collection of different types of fields that hold various types of data.

Drupal 7 developers will remember that in D7, entities had two types of fields, usually referred to as properties and Field UI fields. The former were essentially simple properties on the entity class and were stored in the entity table itself. The latter were fields that were attached to bundles through the UI and had separate database tables.

Things are somewhat similar in Drupal 8 but also very different. First of all, there is a big difference between the fields that belong to content versus configuration entities. Then, as in D7, we still make a distinction between two types of content entity fields: base fields and configurable fields. However, this is not as big as it used to be in D7 as they both have the same foundation.