Switchover
Allows the primary database to switch roles with one of its standby databases.There is no data loss during a switchover. After a switchover, each database continues to participate in the Data Guard configuration with its new role.

[@more@]

Failover
Transitions a standby database to the primary role in response to a primary database failure. If the primary database was not operating in either maximum protection mode or maximum availability mode before the failure, some data loss may occur. After a failover, the failed database no longer participates in the Data Guard configuration.

During any role transition, the amount of downtime required to complete the operation, the potential for data loss, and the effects on other standby databases in the configuration are determined by:

The state of the primary database just before the transition

The state of the standby database selected for the role transition at the time of the transition

If the selected standby database is configured as a physical standby database or a logical standby database

If the role transition is a switchover or a failover

How much redo data remains to be applied to the selected standby database

Whether or not a standby redo log is configured on the standby database

Whether or not redo log files were previously created for the target standby database

The goal is to perform the role transition as quickly as possible with little or no data loss.