My take is web apps in general are designed around the save/submit model (see servlet API). Meaning, we dont go to the backend that often, unless a specific event occurs. Most of the time it is the user clicking the submit or next button. In some cases, if a person selects an item from a select-one menu, we may got to back end if the selection affects other user inputs (i.e states displayed if USA selected).
AJAX has come on the scene to enable web apps to have more of the feel as desktop applications do. Like in GMail when you click a letter in the ‘To’ field , it displays all your contacts (from a backend database most likely) that meet that criteria. Very Outlook like.
The thing with AJAX is that we are not sending the whole Form envelope on each event (small pieces bundled up and sent to server without a submit), we may have multiple seperate connections with the data (increased number of times data going to server but smaller payloads).
Here are some links on AJAX
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