With a app downloadable Google Play or AppStore or a real barcode reader.
Available on iPhone, Android and Blackberry, the most powerful and free i-nigma reader (http://www.i-nigma.com)
The flash of a QR code in three steps
1. Open the app i-nigma
2. Aim the QR Code with the objective of the camera.
3. Note that the QR Code is recognized, you are redirected to the content (web, video, text etc ...)