In a prior life, I followed the enterprise service bus (ESB) and messaging markets pretty closely. Graham Glass (CTO of webMethods) has an interesting post that draws an analogy between the ESB and mechanisms of the human body (bold type added by me to highlight two key underlying capabilities):
The equivalent of an ESB in animals is the nervous system combined with
the circulatory system. When an organ needs to communicate with another
specific organ, it uses the nervous system to send a point-to-point
message. When an organ needs to broadcast a message to other organs
that might be interested, it releases a hormone into the bloodstream to
send a multicast message.
Update (5/9/05): As additional background, an ESB is often used to connect software services (e.g., coarsely-grained software components, for lack of a better word) in a such a way that they can communicate with one another via the backbone network provided by the ESB.