[From OTN]
Physical standby databases provide a rock-solid mechanism for high availability and disaster recovery. In a nutshell, the physical standby database is a physical copy of the source (primary) database, and the redo entries from the primary database are applied to the standby database continuously (or periodically) over the network to keep it in sync. If the primary database fails, the standby database can be quickly activated—a process known as role reversal—enabling applications to connect to the database that was called the standby before and resume processing without a significant delay.
The article continues at
http://www.oracle.com/technology/oramag/oracle/08-nov/o68adg.html
