The following terms are specific to triggers and are important to understand in order to have a successful configuration.
Source Attribute- The attribute that is configured as the source for the trigger.
Target Attribute- The attribute that is configured to store the result of the trigger. The attribute may be in the same class as the source attribute or it make be on a parent class. when an attribute is used as the target of a trigger, it becomes read-only and can only be modified via a trigger, Workflow, or the API.