From Wikipedia: A QR Code is a matrix barcode (or two-dimensional code), readable by QR scanners, mobile phones with a camera, and smartphones. The code consists of black modules arranged in a square pattern on white background. The information encoded can be text, URL or other data.
There are various sites that allows to generate QR codes (as i did with the image for this article).
There are various mobile applications that allows to decode these codes, you can find a good list here; main platforms are supported: Android, iPhone, Symbian, Windows Mobile, Blackberry, Java, etc….
Just shoot the code with your smartphone camera then “scan” it with the application that will decode it.
Finally a funny video made by Google guys about it: