So what is XMLHttp exactly?
XMLHttp allows XML formatted (ideally) data to be passed to a server-side page using client side script. The response from that input can then be read back using a client side script and some actions performed based on that output. Consider this scenario, let’s say you are writing a address verification system. Your boss thinks that the user shouldn’t have to pick the state because just typing in the zip code should automatically select the appropriate state. In the olden days in order to do this you would have to do a Postback on the server and check the zipcode against your database to resolve the state. With XMLHttp once the user types in the zipcode the zipcode is passed to the server to resolve the state. When the result is found the dropdown box for the state is changed to the appropriate state — without ever leaving the page.
Next time I will post some sample XMLHttp code and discuss this further.