Standby
Database 
Use
A standby database is a copy of the active database instance that you update regularly. Import the log backups of the original instance to update the standby instance. The standby instance is in the ADMIN operational state.
If the original instance breaks down, you can start the standby instance immediately, thereby continuing work without long downtimes. You can start the standby instance in a state that corresponds to the state of the original instance at a certain time in the past.
Original Instance and Standby Instance
Activities
Starting the Standby Instance as an Active Instance
See also:
Replication and High Availability
Database
Administration Tutorial, 
Setting Up and
Updating Standby Instances