The image below is a QR Code (wikipedia) for this website:

QR Codes are simple black and white images that can be photographed by camera phones^* -- which will then automatically go to the website that was "encoded" in the image.

To download a QR code (for printing them out or using them on other printed materials) simply right click the image (if you're on a Macintosh, hold down the mouse button until a menu pops up), select "Save image as..." (or "Save picture as..."), choose a folder, and click the "Save" button.