The online backup is a hot backup. A hot backup is a backup performed while the database is open and available for use (read and write activity). Except for Oracle exports, one can only do on-line backups when the database is ARCHIVELOG mode. An online backup places your files onto the Spare Backup data servers.  It is called an online backup because it uses the Internet to transfer your files.

The offline backup is a cold backup. A cold backup is a backup performed while the database is off-line and unavailable to its users. Cold backups can be taken regardless if the database is in ARCHIVELOG or NOARCHIVELOG mode. An offline backup places your files onto a CD/DVD, Hard Disk, or Network Drive.  An offline backup can also be called a local backup.