Sometimes, we want a single action by a user to Trigger other actions. For example, when a person has their Membership Status set to ACTIVE, we would like the system to give them the Member Role and subscribe them to LWVMD newsletters. When a Member reaches the 50-year lifetime membership, we would like their expiration date to be erased.

The system allows us to define TRIGGERS and then to take a certain set of ACTIONS for that Trigger.  You can read more about it at http://drupal.org/node/924538.