History of Entities
The very first thing I learned when started working Drupal was this: "everything is a node." This is what allows you to write a module that works generally on nodes and have it apply to sll sorts of content you have no idea about. Later, I learned that this wasn't quite true. It's true that all content types are descended from notes. Comments, users, taxonomy terms are not nodes. They don't share a common base type. In Drupal 7, they sought to resolve this with another level of abstraction called Entities. So the new truth is Drupal 7 is: everything is an Entity.
