Syntax:
CREATE
    [DEFINER = { user | CURRENT_USER }]
    TRIGGER trigger_name trigger_time trigger_event
    ON tbl_name FOR EACH ROW trigger_body

This statement creates a new trigger. A trigger is a named database
object that is associated with a table, and that activates when a
particular event occurs for the table. The trigger becomes associated
with the table named tbl_name, which must refer to a permanent table.
You cannot associate a trigger with a TEMPORARY table or a view. CREATE
TRIGGER was added in MySQL 5.0.2.

In MySQL 5.0 CREATE TRIGGER requires the SUPER privilege.

The DEFINER clause determines the security context to be used when
checking access privileges at trigger activation time. It was added in
MySQL 5.0.17. See later in this section for more information.

trigger_time is the trigger action time. It can be BEFORE or AFTER to
indicate that the trigger activates before or after each row to be
modified.

trigger_event indicates the kind of statement that activates the
trigger. The trigger_event can be one of the following:

o INSERT: The trigger is activated whenever a new row is inserted into
  the table; for example, through INSERT, LOAD DATA, and REPLACE
  statements.

o UPDATE: The trigger is activated whenever a row is modified; for
  example, through UPDATE statements.

o DELETE: The trigger is activated whenever a row is deleted from the
  table; for example, through DELETE and REPLACE statements. However,
  DROP TABLE and TRUNCATE TABLE statements on the table do not activate
  this trigger, because they do not use DELETE. See [HELP TRUNCATE
  TABLE].

URL: http://dev.mysql.com/doc/refman/5.0/en/create-trigger.html

