Comet describes applications where the server keeps pushing or streaming data to the client, instead of having the browser keep polling the server for fresh content.
As is illustrated above, Comet applications can deliver data to the client at any time, not only in response to user input. The data is delivered over a single, previously-opened connection. This approach reduces the latency for data delivery significantly.
The architecture relies on a view of data which is event driven on both sides of the HTTP connection. Engineers familiar with SOA or message oriented middleware will find this diagram to be amazingly familiar. The only substantive change is that the endpoint is the browser.
While Comet is similar to Ajax in that is asynchronous, applications that implement the Comet style can communicate state changes with almost negligible latency. This makes it suitable for many types of monitoring and multi-user collaboration applications which would otherwise be difficult or impossible to handle in a browser without plugins.