Oracle Data Guard provides the management, monitoring, and automation software infrastructure to create and maintain one or more standby databases to protect Oracle data from failures, disasters, errors, and data corruptions. Data Guard is unique among Oracle replication solutions in supporting both synchronous (zero data loss) and asynchronous (near-zero data loss) configurations. Administrators can choose either manual or automatic failover of production to a standby system if the primary system fails to maintain high availability for mission-critical applications.
There are three types of standby databases:
A Physical standby uses Redo Apply to maintain a block for block, exact replica of the primary database. Physical standby databases provide the best DR protection for the Oracle Database. We1 use this standby database type in this reference architecture
A Logical standby database uses SQL Apply to maintain a logical replica of the primary database. While a logical standby database contains the same data as the primary database, the physical organization and structure of the data can be different
A Snapshot standby database is a fully updatable standby database. Like a physical or logical standby database, a snapshot standby database receives and archives redo data from a primary database. Unlike a physical or logical standby database, a snapshot standby database does not apply the redo data that it receives. The redo data received by a snapshot standby database is not applied until the snapshot standby is converted back into a physical standby database, after first discarding any local updates made to the snapshot standby database.
More information on Oracle Data Guard can be found here.