A QR Code is a matrix code (or two-dimensional bar code) created by Japanese corporation Denso-Wave in 1994.
The “QR” is derived from “Quick Response”, as the creator intended the code to allow its contents to be decoded at high speed.
– from: http://en.wikipedia.org/wiki/QR_Code
I recently got interested in them when I started using Beetight, online bee hive tracking and record keeping software.
It does this nifty thing, where it creates QR code that you can staple (laminated, of course) on each of your hives. When you get to the bee-yard, all you need to do is scan the code and and, viola!, you are at the right place to do hive data entry. Brilliant, in my book.
They suggested BeeTagg to read the labels on the hives (with an iPhone). It has worked quite well for me, do far.
There is also this reader for other kinds of mobile devices: http://reader.kaywa.com/