A QR (quick response) code is a type of barcode that you can scan with a smartphone camera to quickly navigate to a linked URL. A QR code can direct to an online payment website or app, but simply scanning the code will not initiate or authorize a transaction.
NFC (near-field communication) tags are microchips that use radio waves to communicate information, including payment information. Using a technology called credit card tokenization, NFC-enabled devices can be used to complete quick, close-range transactions after two connected devices approve a sale.