There is not just one standard bar code; instead, there are over 400 barcode symbologies that serve different uses, industries and geographic needs. Symbologies can be designed to encode numbers, letters and special characters. There are generally two kinds of barcode symbologies -- discrete and continuous. In a discrete symbology, every character in the bar code can be interpreted individually without referencing the rest of the bar code. In a continuous symbology, individual characters in the bar code cannot be interpreted individually.
Most barcode readers have a feature called "auto-discrimination," which allows them to be configured to automatically recognize and read different barcode symbologies, much the same way a human reader can interpret and read different font types.
See also: bar code reader
Tony Seideman has written about the history of barcodes.