A QR Code (it stands for "Quick Response") is a mobile phone readable barcode.
Version 1 QR Code is a 21x21 array of data elements (25 characters).
The average resolution of present-day camera is about the size of the data modules (dots) on a QR Code of Version 5 or above (37x37).
When creating a QR Code intended for use with mobile phones it's best to stick to Version 4 or lower
(100 characters), and a QR Code symbol of at least 2cm (0.85inches) across.
The largest standard QR Code is a Version 40 symbol that 177x177 modules in
size and can hold up 4296 characters of alphanumeric data (theoretically)
compared to 25 characters for a Version 1 QR Code.