You can relate objects in several different ways. Depending on what you are trying to accomplish, some object association methods are more ideal than others.
Relationship attributes can be used when you want to relate one or multiple objects in one class to an object in another class. You also can use a relationship attribute to relate objects to another object in the same class. This configuration does not support many to many relationships.
Association classes can be used to associate many objects in one class to objects in another class. You cannot relate objects in the same class.
Advanced Associations should be created when two classes need to be associated to each other and additional attributes need to be added in the class that creates the association between the two. In addition, Advanced Associations should be used when you want to associate objects of a class to objects of the same class. Neither of these configurations can be accomplished using an Association Class.