1. A web - services capable infrastructure that supports intelligently directed communication and mediated relationships among loosely coupled and decoupled biz components
2. A standard based integration backbone, combining messaging, web services, transformation, and intelligent routing
In recent years the ESB has become a new segment in the EAI market that attempts to address the intersection of web services and traditional message oriented middleware.
The Microsoft ESB guidance provides architectural guidance, patterns and practice and set of BizTalk Server and .NET components to simplify the development of an Enterprise Service Bus on the Microsoft Platform and to allow developers to extend their own messaging and integration solutions.
I have found some good presentations on ESB overview. Click here to download the presentation files and other details on ESB.