Assuming you had a page with content and a footer, you'd wrap them in a container div, and set the container and the html height to 100%. I put in different colors, so you could see that even thought the content div is short, the footer would still appear at the bottom of the page.
So if this is in the body:
<body>
<div id="container">
<div id="content">header</div>
<div id="footer">footer</div>
</div>
</body>
The style would be:
<style>
html, body {height: 100%;}
#container {height: 100% position:relative;}
#content {background-color:red;}
#footer {background-color:blue; position: absolute; bottom: 0;}
</style>