DCOM, CORBA, and RMI are middleware technologies that enable distributed processing and inter-system communication. DCOM allows components to operate across the Internet by lowering integration costs and improving flexibility. CORBA uses IDL and provides language-neutral communication through its ORB. RMI is fully integrated with Java. Web services use WSDL to describe services, SOAP for messaging, and UDDI as a registry. Socket programming uses sockets as endpoints for communication between systems over TCP and UDP. Message queues store and forward messages to allow asynchronous communication between applications.