A trigger fires when:

120.

a valid event is fired and if more than one table rows are affected.

a valid event is fired, regardless of whether or not any table rows are affected.

only if an UPDATE or DELETE or INSERT event has occured and more than one table rows are affected

only if an UPDATE or DELETE event has occured, regardless of whether or not any table rows are affected.

a valid event is fired, regardless of whether or not any table rows are affected. Valid events include INSERT/UPDATE and DELETE.