Oracle GoldenGate how to

Unlike Active Data Guard, GoldenGate captures primary database changes by reading redo records from disk, transforming those records into a platform independent trail file format, and transmitting the trail file to the target database. GoldenGate maintains a logical replica by converting the trail file into SQL and applying SQL to the target database. The target database is open read-write while synchronization occurs.  While this provides substantial flexibility as a replication solution when compared to Data Guard, care must be taken to insure that the target database is not modified independent of the source, unless it is explicitly desired to do so. 

http://gavinsoorma.com/2010/02/goldengate-concepts-and-architecture/