I find there is always some confusion when talking about Redo, Rollback and Undo. They all sound like pretty much the same thing or at least pretty close.
Redo: Every Oracle database has a set of (two or more) redo log files. The redo log records all changes made to data, including both uncommitted and committed changes. In addition to the online redo logs, Oracle also stores archive redo logs. All redo logs are used in recovery situations.
Rollback: More specifically rollback segments. Rollback segments store the data as it was before changes were made. This is in contrast to the redo log which is a record of the insert/update/deletes.
Undo: Rollback segments. They both are really one in the same. Undo data is stored in the undo tablespace. Undo is helpful in building a read consistent view of data.