A bar code is computerized information encoded in a pattern of black and white stripes. The black and white lines represent 1 and 0 and can be read by light. They carry encoded information -from the membership number of a sports club to the price and stock number of a packet of washing powder in a supermarket. In fact bar codes are used to store data of all kinds.
The code is scanned by a beam of light. When a beam of light is passed over the bar code only the white stripes reflect back the light. This is picked up by a photo-detector which produces a pulse of electricity when it receives light. So the black and white bar code is translated into on/off pulses of electricity. These pulses are fed into a computer for decoding. Now a days laser beams are being used for reading the bar codes.