:A trigger is a specification that the database should automatically execute a particular function whenever a certain type of operation is performed. Triggers can be defined to execute either before or after any INSERT, UPDATE, orDELETE operation, either once per modified row, or once per SQL statement. If a trigger event occurs, the trigger's function is called at the appropriate time to handle the event.[http://www.postgresql.org/docs/8.1/interactive/triggers.html]

: [http://en.wikipedia.org/wiki/Database_trigger Wiki]