A slightly stale bit, but I ran across this piece by Alex Russell which defines Comet, an extension (improvement?) on Ajax style client-side communication. That's my brief summary, but Russell fleshes out the technical differences between Ajax and Comet in more detail. It took me a while to untangle a key figure in his post, but then I realized that the key point of the Comet architecture involves a server autonomously pushing data to a client side application, not just the client making requests and receiving responses.
The hope is that by putting a name to the collective techniques, and identifying specific examples in the wild, momentum and shared experiences can be built. For example, Dojo, a client-side JavaScript toolkit which Russell helps develop, already supports Comet techniques. If you read some followup posts on his blog, looks like there's already quite a bit of action around Comet.