QR code, or Quick Response Code is a two dimensional bar code system invented by Denso, a Japanese Corporation, in 1994. Even though it being a very popularly used alternative to the single dimensional barcode system within Japan, its usage is limited in other parts of the world. The usefulness of a QR code is its ability to store much larger amounts of information and make it possible to be decoded at much greater speeds.
QR codes were designed initially to be used to track parts built in vehicle manufacturing systems or very large scale (the reason to why Denso, a popular vehicle part manufacturer invented it). But, as of today, its usefulness is found in quite different domains, contradictory to the thoughts of the initial developers. QR codes are used in applications related to mobile phones and also as means of an alternate mechanism to store URLs in a universally decodable format.
Bloggers have found it interesting to have a QR code of their blog (some prefer having the code varied by post as well, as you see in my blog), displayed on the widget bar. Fancy looking QR codes are not easy to generate. There are several popular QR code generators including the Google Chart API. Others are, http://qrcode.kaywa.com and http://www.qrcode.es. This blog uses the latter.
Benefits of using QR codes is the possibility to print the image on some form of media (paper, magazine, contact card etc.) and have a camera phone take a photograph of that image and automatically redirect to that location, using a QR code reader application along with a suitable web browser. Thus the usefulness is immense. For instance, if you have a QR code reader, sharing your favorite blog post with a friend is as simple as sending him an image of your QR code generated using the URL.