Here's a snippet from a bigger article written by Mark Rittman over on OTN.
Oracle Data Integrator in Relation to Oracle Warehouse Builder
At this point, regular users of Oracle Warehouse Builder are probably wondering how Oracle Data Integrator relates to it and how it fits into the rest of the Oracle data warehousing technology stack. The answer is that Oracle Data Integrator is a tool that’s complementary to Oracle Warehouse Builder and can be particularly useful when the work involved in creating the staging and integration layers in your Oracle data warehouse is nontrivial or involves SOA or non-Oracle database sources.
For those who are building an Oracle data warehouse, Oracle Warehouse Builder has a strong set of Oracle-specific data warehousing features such as support for modeling of relational and multidimensional data structures, integration with Oracle Business Intelligence Discoverer, support for loading slowly changing dimensions, and a data profiler for understanding the structure and semantics of your data.
Where Oracle Data Integrator provides value is in the initial preparation and integration of your source data, up until the staging area of the data warehouse.
Oracle Data Integrator can integrate and synthesize data from numerous disparate datasources, including Web services and event-based architectures, and, as shown in the figure above, provides a handy graphical interface on top of Oracle Database-specific features such as Oracle Change Data Capture. Once data has been integrated and copied into your data warehouse staging area, Oracle Warehouse Builder can take over and create and populate your operational data store and dimensional warehouse layers.