AJAX :: Using Extenders To Make Client-Side?

I am developing a web application that needs to retrieve, display and write data to an SQL database. I'm trying to use AJAX and Javascripts in order to populate fields, lists and data, and to have button and calendar controls on the page that can update the page without refreshing and losing the data page. Therefore, it's necessary that I find ways to doing as much as possible client-side, without page refreshes.

Now, for security purposes, I've heard that there's a method for retrieving a copy of all relevant data from SQL at loading time, keeping a copy of it, perhaps on a separate page, and send-retrieve-display this data on my page, and then, at Submission time, write this data back to the Server. This is to minimize hits to the Server and to preserve data integrity. I've heard that doing this is called using an Extender, but we can't find examples of how this works so far.