What is the difference between "data-role" and "role" in the following example ?

What is the difference between "data-role" and "role" in the following example ?

http://api.jquerymobile.com/page/

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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>page demo</title>
<link rel="stylesheet" href="//code.jquery.com/mobile/1.4.5/jquery.mobile-1.4.5.min.css">
<script src="//code.jquery.com/jquery-1.10.2.min.js"></script>
<script src="//code.jquery.com/mobile/1.4.5/jquery.mobile-1.4.5.min.js"></script>
</head>
<body>
<div data-role="page" id="page1">
<div data-role="header">
<h1>Page header (optional): Example page</h1>
</div>
<div role="main" class="ui-content">
<h2>Page content</h2>
<p>Page content goes here.</p>
</div>
<div data-role="footer">
<h1>Page footer (optional)</h1>
</div>
</div>
</body>
</html>