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

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 requires the TRIGGER privilege for the table associated
with the trigger. (Before MySQL 5.1.6, this statement requires the
SUPER privilege.)

The DEFINER clause determines the security context to be used when
checking access privileges at trigger activation time.

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 statements on the table do not activate this
  trigger, because they do not use DELETE. Dropping a partition does
  not activate DELETE triggers, either. See [HELP TRUNCATE TABLE].

URL: http://dev.mysql.com/doc/refman/5.1/en/create-trigger.html

