Barcodes can be used for product stickers, price labels, serial numbers and many other purposes. A barcode normally consists of a series of bars and spaces in different thicknesses whereby, depending on the code, the spaces between the bars also hold information.
See also:
Overview of the General 1-D Codes
Overview of the General 2-D Codes
Overview of Post Codes (1-D and 2-D Codes)