A QR Code is a matrix (two-dimensional) barcode. Arbitrary content may be encoded, with URLs being a popular choice :)
Each QR Code contains error recovery information to aid reading damaged or obscured codes. There are four levels of error recovery: Low, medium, high and highest. QR Codes with a higher recovery level are more robust to damage, at the cost of being physically larger.
Copyright (c) 2014 Tom Harwood