QR code
A QR code is the square of black and white dots you point a telephone at: on a poster, on a restaurant table, at the foot of a bill.
What the word means
It carries a short piece of text: a web address, the details of a payment, a name and a telephone number. The telephone reads it and does the rest.
The three large squares in its corners are how a camera finds it. They let the telephone work out where the code is and which way up, from any angle, which is why it reads at a glance and off a badly held page.
What it does on a real document
At the foot of an invoice it holds the account, the amount and the reference of the payment. The customer pays with a camera and a tap, and nothing is mistyped.
Part of the code is spare on purpose, so that it still reads with a corner missing or a coffee ring across it. How much is spare is a choice: more room for damage means a denser code for the same text.
Where it comes from, and why
It was invented in 1994 in Japan, at a company called Denso Wave, for the assembly lines of the motor industry. The striped codes then in use held too little and had to be read one at a time in the right direction, and a production line could not wait for that.
The company chose not to charge for its use, which is the reason it is everywhere and its rivals are not. It has been an international standard since, under the number ISO/IEC 18004.