The following are true about backup:
Full backup: Here all files that have been chosen for backup are backed up, irrespective of wheather the archive bit is set or not set. Archive bit is set (ON) after backup.
Incremental backup: Here only the files that have been created or have changed since the previous full or incremental backup will be backed up. The archive bit is set after a file is backed up. The next Incremental backup will backup files that have changed since previous full or incremental backup.
Differential backup: Here, the files that have changed or created since the last full backup will be backed up. Note that, unlike Incremental backup, the archive bit is not set on a differential backup. The result of this is that the next differential backup will include files that were backed up during earlier Differential backups.