Directly from Oracle's website:
SQL Developer Data Modeler is an independent, standalone product with a full spectrum of data and database modeling tools and utilities, including modeling for Entity Relationship Diagrams (ERD), Relational (database design), Data Type and Multi-dimensional modeling, full forward and reverse engineering and DDL code generation. The Data Modeler imports from and exports to a variety of sources and targets, provides a variety of formatting options and validates the models through a predefined set of design rules.
Oracle SQL Developer Data Modeler can connect to any supported Oracle Database and is platform independent. A read-only SQL Developer Data Modeler Viewer is also available for download. The viewer allows users to open models created in the Data Modeler.