Association is the most general type of relationship and it includes other types as well. If the relationship doesn’t fall into a more specific type, like Aggregation or Composition, it can simply be referred to as an Association.
Example: When a Customer places an Order, the relationship is simply an Association.
Aggregation is a more specific type of association. In an aggregation the children can also be shared with another owner at the same time, and even if the owner no longer exists, the children can still continue their lifetime.
Example: The relationship between a UserGroup and the Users, is an Aggregation. A User can still have meaning in the system even if it doesn’t belong to a UserGroup, so if you delete a UserGroup you won’t delete its Users. On the other hand, the Users can belong to several UserGroups at the same time.
Composition is a more strict type of Aggregation. In an aggregation, the children cannot be shared with a different owner and it doesn’t make sense for the children to exist without their owner. So, you usually want to delete the children if you delete their owner.
Example: The relationship between an Order and the OrderDetails is a composition relationship. The OrderDetail items are valid only as long as there is an Order related to them. When you delete an Order you will delete it’s OrderDetails as well. An OrderDetail associated to a particular Order cannot belong to a different Order.