Can you please explain me How actually QR codes are created and How they are tracked. Generated codes are stored in a DB.

I am actually confused about how QR code reader application works. If I create a site where users can generate QR codes. Every generated QR code will be saved in my site DB. Then, will I have to write an application too? An application that will read read QR code images from an image and will act accordingly or will an QR code reader app will do the job?

regards,
aqif