A barcode is a printed label containing machine-readable data in the form of vertical bars of generally equal height but in whose variable width and spacing are encoded the frequencies or values of the data represented. The data on the label can be read by an optical scanning device into an attached computer. In libraries, barcodes are used to identify books for circulation and inventory, and to link a borrower's library card to the appropriate patron record in the library's computerized circulation system.
Over time, some barcodes schemes have developed into two dimensional systems using various shapes.
 Further reading
- The Original Barcode FAQ— from Azalea.com