The long-polling Comet technique is a technique that optimizes traditional polling to reduce latency.
Traditional polling sends an XMLHttpRequest to the server in fixed intervals. For example, open a new XMLHttpRequest every 15 seconds, receive an immediate response, and close the connection.
Long-polling sends a request to the server, but a response is not returned to the client until one is available. As soon as the connection is closed, either due to a response being received by the client or if a request times out, a new connection is initiated. The result is a significant reduction in latency because the server usually has a connection established when it is ready to return information to return to the client.
Greg Wilkins recently compared long-polling to traditional polling in the article Comet is Always Better Than Polling.