Oracle Data Guard forms an extension to the Oracle RDBMS. It aids in establishing and maintaining secondary "standby databases" as alternative/supplementary repositories to production "primary databases". A Data Guard Configuration includes a production database, referred to as the primary database, and up to 30 standby databases.
Primary and standby databases connect over TCP/IP using Oracle Net Services. Data Guard automatically sync the primary database and all of it's standby databases.
The "Oracle Active Data Guard" option, an extra-cost facility, extends Oracle Data Guard physical standby functionality in Oracle 11g configurations. It allows read-only access on the standby node at the same time as applying archived transactions from the primary node.