UNDO = 9i and above
rollback = lower than 9i (but you can use rollback instead of
UNDO in 9i even though it is not recomended)
UNDO turns rollbacks segments into auto-management (kind of).
You only need one UNDO tablespace which you can assign multiple
Also, you can use the FLASHBACK option with UNDO which means you
can retrieve committed deleted data. You cannot do this using rollback segments.
All-in-all, once you start using UNDO you almost never have to worry
about it (assuming you set to auto-extend or leave enough segment space in the UNDO)
you can lead someone to something but they will never learn anything ...