There is a mystique surrounding bar codes which intimidates many people. Let's eliminate it quickly. The bar code usually doesn't contain descriptive data, (just like your social security number or car's license plate number doesn't have anything about your name or where you live). The data in a bar code is just a reference number which the computer uses to look up associated computer disk record(s) which contain descriptive data and other pertinent information.

For example, the bar codes found on food items at grocery  stores don't contain the price or description of the food item; instead the bar code has a "product number" (12 digits) in it. When read by a bar code reader and transmitted to the computer, the computer finds the disk file item record(s) associated with that item number. In the disk file is the price, vendor name, quantity on-hand, description, etc. The computer does a "price lookup" by reading the bar code, and then it creates a register of the items and adds the price to the subtotal of the groceries purchased. (It also subtracts the quantity from the "on-hand" total.)

Another example of bar code data might be in a quality reporting application, the bar code may have only a single  digit in it, but it may be titled "Failed Vibration Test". The computer  associates the single digit with the test result.

A bar code is a series of varying width vertical lines (called bars) and spaces. Bars and spaces together are named "elements". There are different combinations  of the bars and spaces which represent different characters.

When a bar code scanner is passed over the bar code, the light source from the scanner is absorbed by the dark bars and not reflected,  but it is reflected by the light spaces. A photocell detector in the scanner  receives the reflected light and converts the light into an electrical signal. As the wand is passed over the bar code, the scanner creates  a low electrical signal for the spaces (reflected light) and a high electrical  signal for the bars (nothing is reflected); the duration of the electrical signal determines wide vs. narrow elements. This signal can be "decoded" by the bar code reader's decoder into the characters that the bar code represents. The  decoded data is then passed to the computer in a traditional data format.