A QR code is  a machine-readable 2D image, similar to a barcode.  They can be read by scanners or by applications on smart phones, which then directs the user to any URL that the QR code owner desires.

A QR code is a really simple way to link physical content (e.g. newspaper, brochure, poster) to online content. No need to type or search. Simply scan the code and your device will take you to the online content. They can also link to contact details, a phone number, email or plain text. In these cases, you can think of QR Codes as "Digital Business Cards".