CSS Media Queries were originally a proposal submitted to the CSS Working Group by Opera Software and are now implemented in Firefox 3.5. In short, Media Queries extend the media declaration attached to a stylesheet to allow matching based on the rendering device's intrinsic properties. 

Let's take a link element declaring a stylesheet inside an HTML document:

<link rel="stylesheet" type="text/css" href="style.css"
Now imagine you want this stylesheet to apply to the document if and only if the width of the content window is less than 300 pixels... CSS Media Queries make it simple to declare:
<link rel="stylesheet" type="text/css" href="style.css"
media="screen and (max-width: 300px)">

Available properties include viewport's width and height, device's width and height, orientation (portrait or landscape),  viewport's aspect ratio, device's aspect ratio, colormap, resolution and type of device.

It's then very easy to have one single web page ready for consumption on a wide variety of devices, ranging from mobile devices to monochrome tty displays.

Click here to see the demo and don't forget to resize your window from 100px-wide to 900px-wide to see it in action (WARNING: this link opens a new window without toolbars because toolbars block the size of a viewport above roughly 400px...)