scott@ORA92> -- starting data:
scott@ORA92> select * from a
  2  /

        ID NAME            NATIONALITY
---------- --------------- ---------------
         1 Godwin          USA

scott@ORA92> select * from b
  2  /

        ID G COURSE          NATIONALITY
---------- - --------------- ---------------
         1 A SQL and PL/SQL  USA

scott@ORA92> -- trigger:
scott@ORA92> create or replace trigger a_bur
  2    before update on a
  3    for each row
  4  begin
  5    update b
  6    set    nationality = :new.nationality
  7    where  id = :new.id;
  8  end a_bur;
  9  /

Trigger created.

scott@ORA92> show errors
No errors.

scott@ORA92> -- test:
scott@ORA92> update a set nationality = 'FRANCE'
  2  where  name = 'Godwin'
  3  /

1 row updated.

scott@ORA92> -- results:
scott@ORA92> select * from a
  2  /

        ID NAME            NATIONALITY
---------- --------------- ---------------
         1 Godwin          FRANCE

scott@ORA92> select * from b
  2  /

        ID G COURSE          NATIONALITY
---------- - --------------- ---------------
         1 A SQL and PL/SQL  FRANCE

SQL> CREATE TABLE source_table( id   NUMBER
  2                           , name VARCHAR2(64)
  3                           , nat  VARCHAR2(2)
  4                           , CONSTRAINT st_pk PRIMARY KEY (id)
  5                           )
  6  /

Table created.

SQL> 
SQL> CREATE SEQUENCE aud_seq 
  2    START WITH 1 
  3    INCREMENT BY 1 
  4    MINVALUE 1 
  5    CACHE 20 
  6    NOCYCLE 
  7    NOORDER 
  8  /

Sequence created.

SQL> 
SQL> CREATE TABLE audit_table( aud_id    NUMBER
  2                          , tab_name  VARCHAR2(64)  NOT NULL
  3                          , operation VARCHAR2(1)
  4                          , tab_pk    NUMBER        NOT NULL
  5                          , name      VARCHAR2(64)
  6                          , nat       VARCHAR2(2)
  7                          , timestamp DATE          NOT NULL
  8                          , CONSTRAINT aud_pk PRIMARY KEY (aud_id)
  9                          )
 10  /

Table created.

SQL> 
SQL> CREATE TRIGGER st_bruid
  2    BEFORE INSERT OR UPDATE OR DELETE 
  3    ON source_table 
  4    FOR EACH ROW
  5  DECLARE
  6    v_id NUMBER;
  7  BEGIN
  8    -- generate primary key
  9    SELECT aud_seq.nextval INTO v_id FROM dual;
 10  
 11    IF INSERTING THEN
 12      INSERT INTO audit_table
 13      VALUES ( v_id  -- aud_id
 14             , 'SOURCE_TABLE' -- tab_name
 15             , 'I'            -- operation (I)nsert
 16             , :NEW.id        -- tab_pk
 17             , :NEW.name      -- name from source_table
 18             , :NEW.nat       -- nat(ionality) from source_table
 19             , SYSDATE        -- timestamp (use timestamp datatype on 9i)
 20             );
 21    ELSIF UPDATING THEN
 22      INSERT INTO audit_table
 23      VALUES ( v_id  -- aud_id
 24             , 'SOURCE_TABLE' -- tab_name
 25             , 'U'            -- operation (U)pdate
 26             , :NEW.id        -- tab_pk
 27             , :NEW.name      -- name from source_table
 28             , :NEW.nat       -- nat(ionality) from source_table
 29             , SYSDATE        -- timestamp (use timestamp datatype on 9i)
 30             );
 31    ELSIF DELETING THEN
 32      INSERT INTO audit_table
 33      VALUES ( v_id  -- aud_id
 34             , 'SOURCE_TABLE' -- tab_name
 35             , 'D'            -- operation (D)elete
 36             , :OLD.id        -- tab_pk
 37             , :OLD.name      -- name from source_table
 38             , :OLD.nat       -- nat(ionality) from source_table
 39             , SYSDATE        -- timestamp (use timestamp datatype on 9i)
 40             );
 41    END IF;    
 42  END;
 43  /

Trigger created.

SQL> INSERT INTO source_table values(1, 'Peter','GB');

1 row created.

SQL> UPDATE source_table SET nat = 'US' Where name = 'Peter';

1 row updated.

SQL> INSERT INTO source_table values(2,'Mark','FR');

1 row created.

SQL> INSERT INTO source_table values(3,'Frank','GB');

1 row created.

SQL> DELETE FROM source_table WHERE id = 2;

1 row deleted.

SQL> commit;

Commit complete.

SQL> SELECT *
  2    FROM audit_table
  3   ORDER BY timestamp;

   AUD_ID TAB_NAME             O    TAB_PK NAME       NA TIMESTAMP
--------- -------------------- - --------- ---------- -- --------------------
        1 SOURCE_TABLE         I         1 Peter      GB 17-AUG-2004 08:23:29
        2 SOURCE_TABLE         U         1 Peter      US 17-AUG-2004 08:23:33
        3 SOURCE_TABLE         I         2 Mark       FR 17-AUG-2004 08:23:38
        4 SOURCE_TABLE         I         3 Frank      GB 17-AUG-2004 08:23:43
        5 SOURCE_TABLE         D         2 Mark       FR 17-AUG-2004 08:23:46

SQL>