QR (Quick Response) are two-dimensional barcodes which are easy to set up, as in essence in it is a barcode with a URL attached.
The QR code can link to any page the owner specifies, saving the need for manual entry of a URL.
In order to read QR codes you will need to install a scanning app on your smartphone.
To uses these apps you point the phones camera at the QR code as if you were taking a photograph. The code is then translated into a URL which can be quickly opened on your smart phone browser without typing in the full URL.
For example the QR code below will take you to my page optimised for mobile phones.
To generate your own code visit http://qrcode.kaywa.com/