CORBAs goal is to address interoperability at various levels. There is a history to this.
In the early versions of CORBA, interoperability between platforms and programming languages was addressed. This included the standardization of IDL and the mapping of IDL to a programming language. While a client and server developed with the same vendors ORB could talk to one another, a client and server developed with different vendors? ORBs were not likely to interoperate.
CORBA 2.0 introduced interoperability between different ORB vendors. This resulted from the introduction of a standard wire protocol called General Inter-ORB Protocol (GIOP), and the incarnation for GIOP for the internet, known as Internet Inter-ORB Protocol (IIOP). So CORBA 2.0 compliant ORBs will interoperate. This means a client using ORB vendor A can talk to a server using ORB vendor B.
Interoperability is actually a broader issue than just have ORB vendor A talking to ORB vendor B. Fuller interoperability means that various services interoperate. For example, while a CORBA object can talk to a DCOM object via a protocol bridge, can the CORBA Transaction Service talk to the Microsoft Transaction Service to have a seamless transaction between systems? This broader interoperability at the service level is being addressed now.