UNDO tablespace and REDO log files are totally different concept.
UNDO tablespace is a new concept introduced from Oracle 9i. Prior to 9i, the undo is implement as rollback segments. Therefore, you can think that UDO tbs = ROLLBACK segments (there are big difference between them, but their purposes are same)
RDEO logfiles are solely used for recovery.
In the 031's student guide, these two topics are covered in different chapters. For details, please read those chapters.
