Dynamic server response

Hi. Can anybody tell me how I can make the server send out data to connected users upon an event? The case is say 2 people are sending messages, chatting, instead of having ajax calls every second is there a way to tell the server when to display new messages? I believe in order for this to happen the browser has to not close the connection to the server but as my nickname states I'm not that good and I don't know how to do this hence I'm asking