If you’ve seen this several times but you don’t what it is, then you should take a look at Windows Live Barcode.
It’s actual name is Quick Response Code or QR Code in short. A QR Code is a grid of black and white blocks and it encapsulates a piece of text information by arranging the blocks into some unique pattern. Then it is convenient to exchange this piece of information by reading it using a QR Code scanner, instead of typing the text manually.
* QR Code® is a registered trademark of Denso Wave Inc.