Please explain - rollforward & rollback.

• Rollforward refers to the process Oracle goes through to apply changes contained in the redo log files (both online and archive). The database SCN is moved forward within the blocks of the datafile that are changed within the redo log vectors. Roll forward occurs during database tablespace or datafile recovery and during crash recovery.
• Rollback is the process of undoing uncommitted database transactions. The blocks copied to the rollback segments during transactions as a copy of the block for other transaction to read.When the instance aborts the undo information in the redo log files must be applied to the database during the roll forward process of recovery. Therefore during recovery the database must roll forward and roll back.
•  Roll forward is applied on Redo log files as Rollback is applied on undo log files.