QR codes are two-dimensional barcodes that can be read by many cell phones and smartphones. The codes, which are small squares with black and white patterns, appear in a variety of places, such as magazine and newspaper ads. A QR code is used to encode some sort of information, such as text or a URL.
The "QR" in QR codes stands for "quick response," as the codes are designed to be read quickly. QR codes can be read by dedicated QR code readers and by some cell phones. To read a QR code, your cell phone will need a camera -- so it can snap a picture of the code -- and a QR code reader. You can find many free QR code readers in the various app stores for different phone platforms.
Once your cell phone reads the code, the information that it stores is shared with you. You may be taken to a URL where you can watch a movie trailer, or you may be given details about the company you saw advertised. You may even be presented with a coupon for a local business.