What is an Enterprise Service Bus?

An Enterprise Service Bus (ESB) is an architecture with a set of rules and principles for integrating numerous applications together over a bus-like infrastructure. ESB products enable users to build this type of architecture, but vary in the way that they do it and the capabilities that they offer. 

ESB architecture integrates different applications by putting a communication bus between them and then enabling each to talk to the bus. This allows connected applications  to communicate without dependency on or knowledge of other systems on the bus.