You'll want to look at jQuery events too then. In particular the click event.
Also, be aware that jQuery code generally needs to be loaded after the DOM is finnished loading. It has a special event you attach to the document to do just that.
So, a complete example might look like:
<!DOCTYPE html>
<html>
<head>
<script src="http://code.jquery.com/jquery-latest.js"></script>
</head>
<body>
<span id="foo">click here</span>
<div id="bar"></div>
</body>
<script>
$(document).ready(function() {
$('#foo').click(function() {
$('#bar').load("/somefile.php");
});
});
</script>
</html>