An Enterprise Service Bus (ESB) is a standards-based, 
Service Oriented Architecture (SOA) backbone, capable of 
connecting applications through service interfaces. By 
combining Messaging, Web Services, XML, Data Transformation 
and Management, an ESB can reliably connect, mediate and 
control all communications and interactions between 
services.

An esb is a very important component of SOA. Imagine all 
your web services built on different technologies 
(java,.net,mainframe)..interacting with each other using 
xml for data transmission..One doesn't have to worry about 
the schema transformation,protocol switching or message 
enrichment..the esb does it all for you ..

it basically means that the services that you use are 
reusable.One service does not depend upon the other.A 
change in a .net service shall not affect another service 
developed in java.All the components that is needed for an 
application must not be tightly coupled so that a single 
change in some class changes all other parts of the 
application.Instead break the whole application into small 
components ,expose them as web services and orchestrate .