some links about ROWNUM

http://www.dbforums.com/oracle/988716-rownum-order.html

http://www.orafaq.com/wiki/ROWNUM

http://docs.oracle.com/cd/B19306_01/server.102/b14200/pseudocolumns009.htm

http://www.oracle.com/technetwork/issue-archive/2006/06-sep/o56asktom-086197.html

one useful use I learned from Tom Kytes' article is pagination.  e.g the follow sql returns 2nd and 3rd rows.

 select *
  from ( select /*+ FIRST_ROWS(5) */
  a.*, ROWNUM rnum
      from ( select * from t order by b) a
      where ROWNUM <4 )
where rnum  >1 ;