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Dataguard has two aspects:
1) it is a marketing label, to redress an old feature in new clothes
2) it is an interface on top of standby database, automating failover and switchover operations.
IMO, if you don't use the Dataguard OEM module, and/or the DGMGRL command line interface, you are not using Dataguard, but standby database only.
Senior Oracle DBA
The datagaurd meas any dml, ddl, or dcl issued will affect both the databases. In other words you don't need to duplicate the database again and again, Your databases are up to date at any point.
Datagaurd can perform failovers and switchovers transparently without informing the client.
Standby database and datagaurd are 2 different things, because standby database do not performs failovers and switchovers. The client will receive the error.
Datagaurd is one step forward then the standby database.
Years ago.(Oracle 7 era) we used a bunch of scripts to manage standby database, failover and switchover. Each DBA had a similar idea and the world had gajillions of almost similar scripts.
Around the end of Oracle 7 and the beginning of Oracle 8, the Oracle Server Technology team decided to provided some best practices. These got bundled as the original 'Data Guard', which was originally a marketing label for the bundle.
Data Guard has since evolved dramatically. There are a number of different services, executables, and configurations that are included under the Data Guard umbrella. There is a good overview discussed in Chapter 1 of the " Data Guard Concepts and Administration" manual (for 10gR2 it is http://download.oracle.com/docs/cd/B19306_01/server.102/b14239/concepts.htm#g1049956)
Many of us use the terms 'Standby' and 'Data Guard' interchangeably, although that is not correct. Data Guard is Oracle's feature set for Oracle's specific implementation of standby database control available with the Enterprise Edition of the database. (I note that Oracle has started branding backup/recovery and standby capabilities in App Server as RMAN and Data Guard as well - so it's going beyond a feature label into 'marketing brand' territory.)
For standby, you can still script things manually, and for Standard Edition you can purchase a separate product from DBVisit to accomplish a very similar capability.