A QR-code is the most abundant form of a 2D-barcode (wikipedia). QR-codes can be read by e.g. smartphone apps, programs or special scanners. They can provide text information but also links to websites, e-mail adresses, telphone numbers etc.
links to the JOYCE website. Scanned with a smart phone it will provide the web adress http://www.geomet.uni-koeln.de/joyce/ and eventually directly direct to it. It might be used on poster and talks.
There are lots of websites which allow you to generate QR-codes. The one above has been generated on http://goqr.me/de/#t=url.
Some providers allow to include Logos wich mostly means you have to register and sometimes pay for a designer…