Drupal has below key features.
Entities were introduced in Drupal 7 first time. In Drupal, entity is a general concept . So it may be anything like noun(name, person, palace or a thing).
In Drupal each entity has a purpose to represent/show a specific type of data.
Like the user entity, it displays user information. Content entities display content-related data.
entities can be customized by adding fields to them called bundle.
bundles is a subtypes of an entity type.