Standby database

What is a standby database?
A Standby Database is an exact copy of an operational database on a remote (or local*) server, ready to be used for backup, replication, disaster recovery, analysis, shadow environment and reporting, to name a few applications. A standby database is far superior to a normal backup as it is instantly available in the event of a disaster. To restore a backup takes time, and during the restore time the system is not available. With a standby database there is nothing to restore in the event of a disaster as the standby database is always available. It is possible to switch applications over to the standby database in a matter of minutes to allow business continuity.
Overview of standby database:
Exact copy of main database in a remote (or local*) location.
Is held up to date by applying changes from main database.
In the event of a disaster, standby database becomes active.
Users (or applications) are transferred to standby database to continue operation.

Technical overview of standby database:
Database updates are captured in redo logs.
Updates in redo logs are not permanent. They get overwritten, as there is a set number of redo logs and they written to in a cyclic fashion (when the last one is full, the first one is used again).
A copy is made of the redo log which is then called an archive log. This is a permanent copy.
The archive log is transferred to the standby server.
The transferred archive logs are applied to the standby database.
Standby database is up to date with primary database.