Here is the series of arcticles by Mathieu Carbou about Reverse Ajax, Comet, WebSockets, Event-Driven Architecture etc. I think these articles can help anyone interested in this topic to get started:

Reverse Ajax, Part 1: Introduction to Comet

Reverse Ajax, Part 2: WebSockets

Reverse Ajax, Part 3: Web servers and Socket.IO

Reverse Ajax, Part 4: Atmosphere and CometD

Reverse Ajax, Part 5: Event-driven web development

The key point is that in the moder world everyone wants to develop more interactive application which should react almost instantly to the changes on the server side.

So the focus of Reverse Ajax technologies is adding an ability to send messages/notifications from the server side to the client side without waying for the user to push "Refresh" button in UI.