What Is a QR Code?
Definition
A QR Code (Quick Response) is a two-dimensional barcode that stores data in a square matrix. It was developed in Japan in 1994 for tracking parts in automotive manufacturing and later spread worldwide.
QR codes are defined by the ISO/IEC 18004 international standard. This ensures codes created by different producers are compatible with any standard reader.
You can read a QR code with a phone camera or a tool like the camera barcode scanner and see directly what it carries.
How it works
The three large squares in a QR code's corners (finder patterns) let a reader quickly detect the code and its orientation. That's why QR codes read from any angle.
As the content grows, the code's "version" — its matrix size — increases and the modules (small squares) shrink. A short piece of text yields a small, sparse code, while a long URL produces a larger, denser one.
Some modules are reserved for alignment and timing patterns rather than the data itself; these structures help the code decode correctly even when it's skewed or partly damaged.
Content types
A QR code can hold a URL, plain text, Wi-Fi credentials, a contact card (vCard), email, phone number, SMS, calendar event and location, among others. Whoever creates the code decides what the content will be.
Each content type uses a specific format; for example WIFI:T:WPA;S:Network;P:Password;; for Wi-Fi and the vCard format for contact cards. Phones recognize these formats and prompt the appropriate action (join network, add contact).
To create QR codes in different content types, use the QR code generator, and for many codes at once the bulk QR code generator.
Error correction
QR codes include an error correction mechanism called Reed-Solomon, offering four levels: L, M, Q and H. These allow recovery of roughly 7%, 15%, 25% and 30% of the data respectively.
A higher level lets even a partly dirtied, scratched or logo-overlaid code still be read; the trade-off is a denser pattern with more modules.
If you place a logo in the center, choose Q or H, because the logo covers some modules. Always scan-test the code after printing to confirm it reads.
Static vs dynamic QR
In static QR codes the content is written directly into the code; once printed, its target can't be changed, but it works indefinitely and relies on no server. The QR codes these tools produce are static.
Dynamic QR codes instead carry a short redirect link; the real target is stored on a server and can be changed later. This adds flexibility but depends on a service, and if that service goes down the code stops working.
If you don't need to change the target later, a static QR is a more permanent and privacy-respecting choice.
Security
A QR code is neither safe nor dangerous by itself; it is just a carrier that points to whatever link or data it contains. That's why claiming "this QR code is safe" is misleading.
When scanning unknown QR codes, check the link before opening it and be cautious with shortened URLs and pages asking for login details. For more, see are QR codes safe.
For QR codes you create yourself, mind the source too; as long as you control the content, the risk is low.