It's about "real-time online collaboration". IIRC you can try this out with google docs.

Basically is the server informing all participants about changes, i.e. updating the webpage.

There are different "newer" technologies for this already mentioned in this thread.

Though the "older" Reverse Ajax aka Comet seems to be missing in that list. (we are using it at $work and it works well with browsers already allowing AJAX, but admittedly we don't need to scale this to more than a dozen users )