What is a QR code? These things are everywhere nowadays. Little square barcode-looking things. They're on website, they're on pictures, they're on advertisements -- they're everywhere. So what's a QR code and what do you do with it?
In a nutshell, a Quick Response Code is used to tell your phone to do something. Invented by Toyota in the mid-1990s, they "store" more information than traditional UPC barcodes and work better with languages more complicated than English. (Which is to say, most of them, especially in Asia.)
The way it works is this: Using a scanning app (there's Google's own Goggles app, or plenty of others in the Android Market), your phone's camera scans the code and then interprets it. There are different versions of QR codes, which can contain different amounts of information.
Most often, QR codes are used to link to a website or video or some other online content. Your scanning app should preview the link for you as a safety feature so that you can't be automatically redirected to a malicious (or otherwise untoward) website or video.