I did some research, and this is a basic overview of how a barcode scanner works:
A barcode scanner is composed of three parts: the illuminator, the decoder, and the sensor/convertor. The barcode scanner illuminates the barcode with red light using the illuminator system. The sensor/convertor part of the scanner then detects the reflected light. Once the light is detected, an analog signal is generated. This signal contains varying voltage based on the intensities of the light reflection. The analog signal is converted by the sensor into a digital signal. The digital signal is then interpreted by the decoder. The decoder then sends the information to the computer attached to the scanner.
In the case of QR code scanner, the decoder sends the information to your mobile phone instead of a computer. The app you download for your phone that is a QR code scanner contains the illuminator, which is the red light that runs across the screen when you open the app. The sensor and decoder then work to decode the QR code. Then the decoder sends the information to your phone, and you will be able to see where the QR code was supposed to take you.
The way the scanner reads the reflected light it actually a lot more complicated than the way that I simply described it, but I think it's fascinating that all you have to do is download a free app and all of a sudden your phone is a barcode scanner. The barcode was first patented in 1952, and now almost every person with a smart phone carries around their own barcode scanner with them every day. Since this scanning technology is so easily accessible with most phones, QR codes as a marketing tool seem like they will be around for a while.