As Jeff Potts has explained in his "Working With Custom Content Types in Alfresco" tutorial, Associations come in two flavors: Peer Associations and Child Associations. Peer Associations are just that— they define a relationship between two objects but neither is subordinate to the other. Child Associations, on the other hand, are used when the target of the association (or child) should not exist when the source (or parent) goes away. This works like a cascaded delete in a relational database- Delete the parent and the child goes away.
You need to get familiar with Alfresco content model and terminologies.
Please go through "Working With Custom Content Types in Alfresco" tutorial: