A QR code is a type of barcode that most smartphones can read. "EC level" (EC=error correction) allows to make the code readable even if some portion is lost (but it makes it bigger too). Level 1 (Low) : 7% of characters can be restored. Level 2 (Medium) : 15% of characters can be restored. Level 3 (Quartile) : 25% of characters can be restored. Level 4 (High) : 30% of characters can be restored. Here, you can encode alphanumeric characters (meaning: 0–9, A–Z, space and $%*+-./:) or ISO-5589-1 characters (meaning a whole lot more characters, but they take more room). In theory, the size limit is 4,296 (alphanumeric) or 2,953 (ISO-5589-1) characters with EC level=1, but if you use more than 500 characters, the squares get too small and it gets hard to read. If you want to read the QR code, you will need a smartphone (or a Nintendo DS) and some kind of scanner application (there are plenty for free, do a search for "QR code" on your application store).
