An Oracle database can run in one of two modes. By default, the database is created in NOARCHIVELOG mode. When in NOARCHIVELOG mode the database runs normally, but there is no capacity to perform any type of point in time recovery operations or online backups. Thus, you have to shutdown the database to back it up, and when you recover the database you can only recover it to the point of the last backup. While this might be fine for a development environment, the big corporate types tend to frown when a weeks worth of current production accounting data is lost forever.

You can learn more through this URL:

http://www.dba-oracle.com/concepts/archivelog_archived_redo_logs.htm