The document provides an overview of enterprise service buses (ESBs). It defines an ESB as a software architecture that provides integration services through messaging and standards-based integration. The key capabilities of an ESB are described as routing, transformation, choreography, orchestration, transaction management, and security. The document outlines the typical components of an ESB including a mediator, service registry, choreographer, and rules engine. It discusses how process choreography relates to the mediator and considerations for their relationship. Open source ESB projects like Mule and ServiceMix are also mentioned.