You can very easily and effectively load data using jQuery. Here's some examples
html
<div id="mycontent">
</div>
JS load contents of a .php file
var element = "#mycontent";
var link = "script.php";
$(element).load(link);
JS load contents of a .php file after submitting POST
var username = "test";
$.post("script.php", {
user:username
}, function(responseData)
{
$('#mycontent').html(responseData);
});
And of course you can make functions out of these and call them either using setTimer loops, onclick events for buttons and many other ways.
Keep in mind you need to include jQuery for these to work, and although it's possible using pure JS as well, jQuery is safer and much easier.