Rules allows one to define an alternate action to be performed on insertions, updates, or deletions in database tables and views.
Roughly speaking, a rule causes additional commands to be executed when a given command on a given table/view is executed.
Rules are a Postgresql only feature that is not part of the SQL standard.
Views are implemented in Postgresql via rules. Additional rules can be added to a view to make it updateable.
A common use for a rule on a table is for Notifications to be sent when certain events occur.
See also: