* Entity provide unified way to work with different data units in Drupal.
* Entity is an instance of an entity type defined in Drupal.
* Drupal 7 is all about entities. They are everywhere nodes, users, comments, taxonomy terms.
* Drupal 6 field system was only used on content types. but now, field-able entities makes Drupal more flexible.
* Drupal 8 provides two entity types: 1) Configuration entity 2) Content entity
Entity Types >> Bundles >> Fields >> Entities
Putting this in Object-oriented design/programming terms
An entity type is a base class
A bundle is an extended class
A field is a class member, Property, Variable or field instance
An entity is an object/instance of a base/extended class
1) Configuration Entity (An example in core: image style)
}-> Configuration entities are not fieldable and represent a configuration within a site.
}-> Configuration entities also come with a complete set of CRUD hooks.
2) Content Entity
}-> Content entities are fieldable and can have bundles.
}-> Bundles are controlled through configuration entities.