The DOM (Document Object Model) is a way to represent an XML or an HTML document as a tree of nodes. Using DOM methods and properties, you can access any element on the page, modify or remove elements, or add new ones. The DOM is a language-independent API (Application Programming Interface) and can be implemented not only in JavaScript, but also in any other language. For example, you can generate pages on the server-side with PHP's DOM implementation (http://php.net/dom).
Take a look at this example HTML page:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>
<title>My page</title>
</head>
<body>
<p class="opener">first paragraph</p>
<p><em>second</em> paragraph</p>
<p id="closer">final</p>
<!-- and that's about it -->
</body>
</html>