Corresponding Entity References

"CER keeps reference fields in sync. If you have two entities that refer to each other using Entity Reference (or some other kind of reference field), it saves you the trouble of double-editing your entities in order to have them point at each other. If entity Alice references entity Bob, CER will make Bob reference Alice, and keep the references up-to-date when either entity is updated or deleted."

Here's a great blog post about it by Lindsay at Hook 42: http://www.hook42.com/blog/keep-drupal-entity-references-sync-cer-module