A trigger is a named database object that is associated with a table and that activates when a particular event occurs for the table. Activation of a trigger means execution of a set of commands attached to that trigger. For example, BEFORE trigger for INSERT statements could be used to check the values to be inserted into new rows.
Please note:
You can read more about triggers in Using Triggers topic of MySQL Manual: http://dev.mysql.com/doc/refman/5.1/en/triggers.html
With Database Designer for MySQL you can edit table triggers.
See also:
Diagram Objects: Trigger Editor