The Partners module adds two features to CCK nodereference fields.
1) Bidirectionality. A field can be set to be bidirectional with a nodereference field on another node type (or one on the same node type -- including itself). This means that when node A references node B, a reference will also be created from node B back to node A in the appropriate field. When either side of the reference is deleted, the other side is automatically deleted as well.
2) Intermediaries. Intermediaries are nodes that describe the relationship between two other nodes. For example, you can have Drupal create a Membership node when a User node is linked to a Group node. This can improve data separation. These nodes are automatically deleted when the relationship is broken.