Benefits of an ESB

Source application / service has not to be changed

Business events that happened are published by the source application to the bus.
Typically there is an application interested in this event, so it will consume it as this kind of processing would have been done by a point-to-point communication.
→ If business requirements changes and enterprise landscape grow more and more applications might potentially be interested in that kind of events. So the producing application and already existing consumers do not have to be changed at all.

Easy integration through a DSL

Additional systems that are interested in that event just subscribe to the topic or if there is a dispatcher application in place, that consumes special kind of messages laying on the bus, will then dispatch requests to the corresponding systems.
Integration scenarios are typically configured / implemented with the help of a DSL leading to more maintainability and faster realization.

Further benefits

• Heterogeneous environments get interoperable
• Service virutalization aka location transparency
• Service versioning
• Transaction-based message processing

What is the difference between an ESB, SOA and EAI?

ESB - Enterprise Service Bus

An ESB is an integration platform in that applications that want to communicate with each other have to integrate. It also defines a backbone of your enterprise landscape so that applications and services can easily communicate.

SOA - Service Oriented Architecture

A SOA describes an architecture style in that software resources of an enterprise get accessible and discoverable on network as dedicated and defined services.

EAI - Enterprise Application Integration

An EAI is driven by business needs to achieve a certain business objective by connecting applications inside an enterprise and external partner systems. So it is a concept for integrating business functions with the help of a dedicated IT infrastructure along the value change. As they are provided by different applications and platforms by nature EAI deals with data and business process integration.

So what is the difference?

To make it short: 

SOA is an architecture style based on services, EAI is a concept about connecting applications and services to new valuable services. Whereas the ESB is a concrete method for establishing an integration platform for inter-application communication.

Both SOA and EAI need in their concrete transformation components and the ESB plays an very important role for the realization.