Concepts and Architecture Question:

If database is in Archive Log Mode the only it is possible to perform HOT (Online) Backup, however all transaction entries would be done in Redo File and eventually it will be written to Archive Log File, it also called Archived Redo Log files.Hence by backing up Datafiles/Redo Log Files/Control Files and Archived redo Log Files you can have a consistent backup. Only in caseof Cold backup you dont reuire Archive Redo Log files as database instance is down while performing Cold backup.
For more details you may refer - http://download.oracle.com/docs/cd/B19306_01/server.102/b14357/ch12007.htm