Wikipedia defines a QR code as
"a matrixbarcode (or two-dimensional code), readable by QR scanners, mobile phones with a camera, and smartphones. The code consists of black modules arranged in a square pattern on white background. The information encoded can be text, URL or other data.
Common in Japan, where it was created by Toyota subsidiary Denso-Wave in 1994, the QR code is one of the most popular types of two-dimensional barcodes. QR is the acronym for Quick Response, as the creator intended the code to allow its contents to be decoded at high speed."
"QR Codes can be used to display text to the user, to add a vCard contact to the user's device, to open a URI or to compose a text message or email. Users can also generate and print their own QR Code for others to scan and use by visiting one of several free QR Code generating sites.
QR Codes storing addresses and URLs may appear in magazines, on signs, buses, business cards, or on just about any object about which users might need information. Users with a camera phone equipped with the correct reader application can scan the image of the QR Code to display text, contact information, connect to a wireless network, or open a web page in the phone's browser. This act of linking from physical world objects is known as a hardlink or physical world hyperlinks.
Google's mobile Android operating system supports the use of QR codes by natively including the barcode scanner (ZXing) on some models, and the browser supports URI redirection, which allows QR Codes to send metadata to existing applications on the device. Nokia's Symbian operating system is also provided with a barcode scanner, which is able to read QR Codes, while mbarcode  is a QR code reader for the Maemo operating system. In the Apple iOS is not natively included, but some iTunes Apps, for free, are available with reader and metadata browser URL redirection."
Take a look at his handy little tool, too - http://goo.gl
What other applications can you think of for using QR codes in your marketing?