A linear barcode is a sequence of bars (dark lines) and spaces between the bars (white lines). A linear barcode always starts and ends with a bar, a bar is always followed by a space, and a space is always followed by a bar. The width of the bars and spaces are usually different, and the order of the bars and spaces along with their width carry the information coded by the barcode.
There are many different barcode symbologies (codings) each of which uses different orders and widths. If a barcode symbology distinguishes two different widths for the bars and two different widths for the spaces (thin and narrow), the symbology is binary, otherwise the symbology is non-binary. Many barcode symbologies use checksums or checksum characters.