Setting Up Hot Standby Systems: Overview 
A hot standby system consists of a master database and one or more standby databases that are updated at regular intervals. Master and standby databases together form a cluster that, outwardly, behaves like a single database. If the master database fails, one of the standby databases automatically becomes the new master database.
The main advantage of a hot standby system is the short downtime when errors occur (much shorter than the time it would take to restart the master database, for example).
Caution
A hot standby system does not protect you against database errors and inconsistencies caused by users or applications. To prevent these types of errors, you have to perform regular data and log backups. For more information, see Backing Up Databases: Overview.
Available hot standby solutions:
More Information
Database Administration, Hot Standby System
Database Manager CLI, Managing Hot Standby Systems