What is a QR code?
QR code (abbreviated from Quick Response Code) is the trademark for a type of two-dimensional bar code first designed for the automotive industry in Japan.
Bar codes are optical machine-readable labels attached to items that transmit information related to the item.
Wikipedia: what is a QR-code?
Unlike the older one-dimensional barcode that was designed to be mechanically scanned by a narrow beam of light, the QR code is detected as a 2-dimensional digital image by a semiconductor image sensor and is then digitally analyzed by a programmed processor. The processor locates the three distinctive squares at the corners of the image, and uses a smaller square near the fourth corner to normalize the image for size, orientation, and angle of viewing. The small dots are then converted to binary numbers and validity checked with an error-correcting code.
In Math-Net.Ru QR-codes are used to code URLs of webpages.
Smartphone users can install an app with a QR-code scanner that can read a displayed code and convert it to a URL directing the smartphone's browser,
you can find them in App Store (iPhone, iPad), Google Play Store (Android devices).