An AFTER UPDATE Trigger means that Oracle will fire this trigger after the UPDATE operation is executed.

The syntax for an AFTER UPDATE Trigger is:

CREATE or REPLACE TRIGGER trigger_name
AFTER UPDATE
    ON table_name
    [ FOR EACH ROW ]
DECLARE
    -- variable declarations
BEGIN
    -- trigger code
EXCEPTION
    WHEN ...
    -- exception handling
END;

trigger_name is the name of the trigger to create.

Restrictions:

You can not create an AFTER trigger on a view.
You can not update the :NEW values.
You can not update the :OLD values.
For example:

If you had a table created as follows:

CREATE TABLE orders
( order_id number(5),
 quantity number(4),
 cost_per_item number(6,2),
 total_cost number(8,2)
);

We could then create an AFTER UPDATE trigger as follows:

CREATE OR REPLACE TRIGGER orders_after_update
AFTER UPDATE
    ON orders
    FOR EACH ROW

DECLARE
    v_username varchar2(10);

BEGIN

    -- Find username of person performing UPDATE into table
    SELECT user INTO v_username
    FROM dual;

    -- Insert record into audit table
    INSERT INTO orders_audit
     ( order_id,
       quantity_before,
       quantity_after,
       username )
    VALUES
     ( :new.order_id,
       :old.quantity,
       :new.quantity,
       v_username );

END; 

From www.techonthenet.com