Comet is a web framework in which the server sends data to the client using long held connections. As soon as the client receives the message it opens a connection to the server until a complete message is read. This Architecture enables the server to push data to the client rather then the other way around.
The Architecture depends on Ajax to receive message from server and continue receiving contents from the server in the background while the user is using other features on the page.
A similar architecture is proposed in HTML 5 called server-sent events, which adds new html element event-source and a new data format called DOM event stream.
I believe with the improvement of this architecture lot of barriers for web based applications will be lifted and the difference between web based clients and desktop clients will reduce.
The main applications that will benefit from this architecture will be browser based chat applications, Web Ads delivery, email clients etc