New services like Jot Live and Meebo are built with a style of data transmission that is neither traditional nor Ajax. Their brand of low-latency data transfer to the browser is unique, and it is becoming ever-more common. Lacking a better term, I’ve taken to calling this style of event-driven, server-push data streaming “Comet”. It doesn’t stand for anything, and I’m not sure that it should. There is much confusion about how these techniques work, and so using pre-existing definitions and names is as likely to get as much wrong as it would get right.
Defining Comet
For a new term to be useful, at a minimum we need some examples of the technology, a list of the problems being solved, and properties which distinguish it from other techniques.
– from a DojoToolkit Blog at http://alex.dojotoolkit.org/?p=545 http://alex.dojotoolkit.org/?p=545

http://www.lightstreamer.com/
Commercial Ajax-Comet application.