Large enterprises have multiple applications serving different purposes. And the information is scattered across various applications. Most of the times there is a huge overlap of information between different applications but each application uses its own resource to store information. Not only data but common business processing code might be available redundantly in many applications. When applications working in silos can be interconnected with common business entities acting as service and common data shared between services and applications, it is enterprise application integration (EAI). While it is easy to define here, it is an Herculean task to bring together different applications and deifferent teams to make it happen. Enterprise Service Bus (ESB) is one of the approaches to enable EAI. EAI on wikipedia has more details and also the pitfalls.