ajax server update wonders

i always though ajax was the kind of magic where you provide a callback and then the server sends it all kinds of interesting information, which the client uses all kinds of magic on to provide the information to the user. apparently i was most wrong, as it seems that the format is always the client making a request and the server making one single response to this.. so no pushing data from the server? seems so.. but maybe i need to look at it more later. at least wicket seems to only provide a timed component to make these request on given time intervals. or im just blind and cant find other options. found some similar info on other web pages so.. maybe i will be wiser someday but for now i stick with this belief.