Collections are a special type of object/class used to gather up and manage reference to other objects.
You can read any relationship in two directions. For example, you can say Herman Melville wrote Moby Dick or Moby Dick was written by Herman Melville. You can paraphrase each of these two statements as follows:
This way of articulating relationships highlights the existence of collections. The following two figures show various collections.
You can think of collections as collections of objects or as collections of relationships, each with a source and a target object. The following figures show the ways to think of collections.
The figure to the left shows the collection of books written by Herman Melville as an object collection, while the figure to the right shows the same collection as a relationship collection.
The figure on the left shows the collection of authors of Moby Dick as an object collection, while the figure to the right shows the same collection as a relationship collection. The figures make clear that object collections and relationship collections are essentially the same.