JavaScript is a programming language which runs inside a browser, unlike PHP, but is visible from the website inside the HTML.
<html>
<head>
<title>This is the site's title tag.</title>
<script>
alert("Hello, World!");
</script>
</head>
<body>
<h1>This is a heading</h1>
<p>This is a paragraph of text...</p>
<p>The second paragraph goes here.</p>
</body>
</html>