A QR Code (it stands for "Quick Response") is a mobile phone readable barcode that's been big in Japan forever, broke into Europe a while back, and is now getting traction in USA.
In its simplest sense think "print based hypertext link" – simply encode a URL into the QR Code and then point a mobile phone (or other camera-enabled mobile) at it. If the device has had QR Code decoding software installed on it, it will fire up its browser and go straight to that URL.
But it doesn't stop there – a QR Code can also contain a phone number, an SMS message, V-Card data or just plain alphanumeric text, and the scanning device will respond by opening up the correct application to handle the encoded data appropriately courtesy of the FNC1 Application Identifiers that are embedded in the encoded data.
The technical specifications for a QR Code are set down in the ISO-18004 standard so they are the same all over the world, and the only significant variations from one QR code to another (apart from the data it contains) is the number of modules required to store the data. A Version 1 QR Code is a 21×21 array of data elements with the array increasing in size by 4 modules for each increase in version number. The largest standard QR Code is a Version 40 symbol that 177×177 modules in size and can hold up 4296 characters of alphanumeric data (theoretically) compared to 25 characters for a Version 1 QR Code.
While there is still a lot of scope for improvement, the resolution of average present-day camera-enabled portable devices is such that the size of the data modules (dots) on a QR Code of Version 5 or above (37×37) presents a real risk of incorrect decoding of the symbol by the device. When creating a QR Code intended for use with mobile phones and PDA's it's best to stick to Version 4 or lower, and a QR Code symbol of at least 2cm (0.85inches) across.
To make things a bit more robust, the QR Code also contains its own error correction data, internal orientation calibration and self-alignment markers. In this way it doesn't matter whether the QR code is upside down or wrapped around a curved surface, the message will still get through.
For more information about QR Codes, try these sites: