QR codes or Q uick R esponse codes are codes which are designed in a way that they are easily readable by computer programs via a camera. The purpose of qr codes or actually any bar code is the link they do create between the physical and the digital world.
The codes can vary from simple to complex (details on Wikipedia) and do contain some sort of data. This data can be of multiple types usually a webpage or a simple code. The real power of qr codes evolves from the combination with personal and mobile devices. So that often an advertisement poster holds a qr code with a link to the ad campaign website with follow-up information.
The problem for most use cases is that today most of the people do not know what a qr code is and more eminent how these are working. So as soon as you decide to go with qr codes you should be aware that you do loose most of the people unless you do also provide the information in plain text (in this example the url of the ad campaign).
So some people put qr codes on their business card, which already holds all the information, so as receiver you can easily take your mobile device and scan the code, rather than typing everything from the business card once back in the office.
The same way today you find share links for facebook, twitter.. on every website, but how do you bring that on printed poster or static screens. Well with qr codes you can bridge this barrier and take advantage of both worlds, digital and physical.
So whenever you are working on something and you are facing the question how do i bridge from physical to digital you might consider qr codes as one possible answer.