Bar codes provide a simple and inexpensive method of encoding text information easily read by electronic readers. Bar coding also allows data to be collected rapidly and with extreme accuracy. A bar code consists of a series of parallel, adjacent bars and spaces. Predefined bar and space patterns or "symbologies" are used to encode small strings of character data into a printed symbol. Bar codes can be thought of as a printed type of the Morse code with narrow bars (and spaces) representing dots, and wide bars representing dashes. A bar code reader decodes a bar code by scanning a light source across the bar code and measuring the intensity of light reflected back by the white spaces. The pattern of reflected light is detected with a photodiode which produces an electronic signal that exactly matches the printed bar code pattern. This signal is then decoded back to the original data by inexpensive electronic circuits. Due to the design of most bar code symbologies, it does not make any difference if you scan a bar code from right to left or from left to right.

The basic structure of a bar code consists of a leading and trailing quiet zone, a start pattern, one or more data characters, optionally one or two check characters and a stop pattern.

There are a variety of bar code encoding schemes or "symbologies", each of which were developed to fulfill a specific need in a specific industry. Several of these symbologies have matured into de-facto standards that are used universally today throughout most industries.

The different symbologies have different capabilities for encoding data. For example the UPC symbology used to identify retail products always contains 12 numeric digits whereas the general purpose Code 39 or Code 128 bar code symbologies can encode variable length alphanumeric data up to about 30 characters in length. These types of bar codes are called "linear symbologies" because they are made up of a series of lines of different widths. Most commercially available bar code scanners are able to read all of the different linear bar code symbologies therefore you do not need different readers for different types of bar codes.

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