QR codes are a 2 dimensional bar code. Unlike the bar codes you find on products in a shop which have a series of lines to encode data, QR codes are made up of a grid of black and white squares. They are becoming increasingly popular lately, you may have seen them on the side of cans to be used in promotions or similar.
There are many types of 2D bar codes, but QR codes are one of the more popular formats. You can use them to encode a wealth of different things, for example text, a hyperlink or even wireless network access. This data can then be accessed by scanning or photographing the code using a camera or phone. Linking real world physical objects to online/computer content is called hardlinking. Generating codes is as simple as inputting your data into a form online, and he resulting image can be placed anywhere you can think of: on a business card, on a poster, even on clothing. QR code readers are available on many different devices, for example the iPhone.