Sometimes your domain objects have one-to-many or many-to-many relationships with other objects.  For instance, your Customers might have collections of Orders associated with them.  The simplest way to model this is to expose a List<Order> field or property off of your Customer class.  Unfortunately, this has a number of negative consequences because of how it breaks encapsulation.  Fortunately, there are some effective ways to shield yourself from these issues while still exposing the collection's data to your class's clients.