There are several ways to solve the problem of browsers caching AJAX requests. You can use a 'cache buster' parameter, or you can set the response headers server-side. This article discusses these two options.
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How to stop Internet Explorer from caching AJAX requests
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Using AJAX to update the time on a web page from the server
In an attempt to better explain how you can use AJAX to fetch some data from the server and display it on the web page, I have provided a self contained PHP file.