EAI is a very broad term that describes the sharing of information, application services and processes between systems existing within the same enterprise. There are many components to an EAI solution including information exchange and transformation, business process integration, portals, use of standards (e.g., JCA, XML, etc.), use of middleware technology, you get the idea.
Web services fall in the domain of EAI as both an enabling technology and set of standards (e.g., UDDI, WSDL, SOAP), but I would not use the terms interchangeably. For instances, Web services are to EAI what JDBC is to database design and management, and what Java is to application development. One is a concept (EAI), the other is an approach and technology (Web services).
Web services allow EAI architects to integrate many back-end systems through the creation of loosely coupled composite applications. Distributed objects perform a similar trick (albeit they are more tightly coupled). Web services is a technology and set of standards existing in the application service-oriented approach to EAI, which is only one of many approaches.
This was first published in January 2002