What is an Enterprise Service Bus (ESB)?
An Enterprise Service Bus (ESB) is a distributed infrastructure used for enterprise integration. It consists of a set of service containers, which integrate various types of IT assets. The containers are interconnected with a reliable messaging bus. Service containers adapt IT assets to a standard services model, based on XML message exchange using standardized message exchange patterns. The ESB provides services for transforming, orchestrating, mediating and routing messages as well as the ability to centrally administer the distributed system. Note: Read a good conceptual overview of an Enterprise Service Bus on Wikipedia.