A QR Code is a two-dimensional barcode that is readable by smartphones. It allows to encode over 4000 characters in a two dimensional barcode. QR Codes may be used to display text to the user, to open a URL, save a contact to the address book or to compose text messages. "QR Code" is a registered trademark of DENSO WAVE INCORPORATED.
To read QR Codes with your smartphone, you need an appropriate software installed on your phone. For Android-based devices, you can use Barcode Scanner by ZXing. On iOS-Devices like iPhones there are also QR Code readers available on the AppStore, for Example i-nigma. On Firefox OS try QR Code Scanner. On Symbian devices, you can use Mobiletag barcodes reader for example.
If you use a modern web browser (recent versions of Firefox or Chrome) and have a webcam, this site allows you to scan QR Codes with your web browser. You can use this to send a link from a phone to your desktop browser for example.