A BEFORE UPDATE Trigger means that Oracle will fire this trigger before the UPDATE operation is executed.

The syntax for an BEFORE UPDATE Trigger is:

CREATE or REPLACE TRIGGER trigger_name
BEFORE 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 a BEFORE trigger on a view.
You can 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),
 updated_date date,
 updated_by varchar2(10)
);

We could then create a BEFORE UPDATE trigger as follows:

CREATE OR REPLACE TRIGGER orders_before_update
BEFORE UPDATE
    ON orders
    FOR EACH ROW

DECLARE
    v_username varchar2(10);

BEGIN

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

    -- Update updated_date field to current system date
    :new.updated_date := sysdate;

    -- Update updated_by field to the username of the person performing the UPDATE
    :new.updated_by := v_username;

END;

From www.techonthenet.com