There are two types of Standby databases:
1, Physical standby database
block-for-block basis
the physically identical with the primary database
user recovery technology
2, Logical standby database
shares the same schema definition withe the primary database
executing sql statements on the standby database
use logMiner technology

There are three types of services provided with Data Guard:
1, redo transport services
2, log apply service: including redo apply and SQL apply
3, Role-management services

Oracle Data Guard supports two role-transition operations:
1, Switchover
2, Failover

Oracle Data Guard Data Protection Modes:
1,Maximum Protection
2,Maximum Availability
3,Maximum Performance

Benefits of Implementing Oracle Data Guard:
1,You can use a logical standby for real-time reporting and the physical standby database for point-in-time reporting.
2,Logical standby database is open and ready for reporting at all times.

Note:Standby database can use a different directory structure from the primary database.

On the primary database,Data Guard redo transport services use the following processes:
1,Log Writer(LGWR) process
2,Archiver(ARCn) Process
3,Fetch archive log(FAL)

Note:You can configure a primary database to ship redo information to a single standby database by using either LGWR or ARCn,but not both.

On the standby database,Data Guard log apply services use the following processes:
1,Remote file server(RFS) process
2,Archiver(ARCn) process
3,Managed recovery process(MRP)
4,Logical standby process(LSP)

Standby Redo Logs:
A standby redo log is required to implement:
1, The maximum protection and maximum availability levels of data protection.
2,Real-time apply
3,Cascaded redo log destinations

Standby redo logs are recommended for maximum performance data protection mode
Unless you are using the real-time apply feature,standby redo logs must be archived before the data can be applied to the standby database.
The standby archival operation occurs automatically.

The Data Guard physical standby Redo Apply architecture consists of:
A production(primary) database,which is linked to one or more standby databases(up to nine) that are identical copies of the production database.
--The limit of nine standby databases is imposed by the LOG_ARCHIVE_DEST_n parameter.In Oracle Database 10g,the maximum number of destinations is 10. One is used as the local archive destination,leaving the other nine for uses such as the standby database.
Note: You can use the Cascaded Redo Log Destination feature to incorporate more than nine standby databases in your configuration.
--The primary database is open and active.The standby databases are either in recovery mode or open in read-only mode,but not both.
--Redo is applied to each standby database by using standard Oracle recovery techniques.

Logical Standby Database: SQL Apply Architecture
Instead of using media recovery to apply changes(as in the physical standby database configuration),archived redo log information is transformed
into equivalent SQL statements by using LogMiner technology.These SQL statements are then applied to the logical standby database.The logical
standby database is open in read/write mode and is available for reporting capabilities.