Difference between Rollback & Undo Segments

Till Oracle 8i the undo that was generated, used to be handled by the Rollback tablespace, which was dictionary managed. In this case we have to first create a Rollback tablespace, then (according to our needs) create rollback segments and assign it to the rollback tablespace.

Now, from Oracle 9i the new concept of undo tablespace is introduced, which helps the DBAs in the following ways:

1) It is locally managed.
2) The undo segments are created by oracle itself ( whereas in the case of rollback segments we create it).
3) The number of undo segments are generated by oracle itself.

The purpose of undo segment and rollback segment is the same except the creation and maintenance part. Both will do the same functionality,but in oracle 9i they made it simplify, because if rollback segments are used, we need to put them online in init.ora and needs to take care space management etc.

So in oracle 9i they introduced undo tablespaces on that we can create undo segments and space management will be take care by oracle managed files ,so its reduces the burden of DBA.