Recently, one of my clients asked me what the square barcode looking graphic was in the back window of my car. I told him that it was a Quick Response (QR) code for Geeks in Phoenix on Google Places. The blank look on his face told me I needed to tell him more about them.
QR code for Geeks in Phoenix on Google Places
QR code for Geeks in Phoenix contact information
A QR code is a two-dimensional matrix barcode that is readable by barcode scanners in smartphones (Android, iPhone, etc.). It's a square pattern of black modules on a white back ground. It can be encoded with contact information, text, url, etc.. Here's a quote from Wikipedia on QR codes:
Although initially used for tracking parts in vehicle manufacturing, QR codes are now used in a much broader context, including both commercial tracking applications and convenience-oriented applications aimed at mobile phone users (known as mobile tagging). 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 an email or text message. Users can also generate and print their own QR codes 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 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, a QR code reader is not natively included, but over 50 free Apps are available with reader and metadata browser URI redirection capability.
For more information about QR codes, just follow the links below.
QR code - Wikipedia
Google ZXing QR Code Generator