Concepts and Architecture Question:

Rollback segments are used to get data blocks to the original state when user issues a 'rollback' statementgive consistent data to a user, hiding uncommitted changes made by other users/sessionsto undo uncommitted changes during database recovery. After applying all changes recorded in the redo logs, Oracle uses rollback segments to undo uncommitted changes. Since rollback segments are stored in the database buffers, they are automatically protected by the redo log.