Bar codes are one-dimensional bar codes. The information is located in the X-axis and is represented by bars, gaps and their widths.
There are bar codes that use only two bar and gap widths, these are the two-width codes, while others use multiple bar/gap widths. These barcodes are called multi width codes. The difference between the two barcodes is the density of information.
Two-width codes have a lower information density. They use width ratios of 2:1 and 3:1 for the two different widths. This means that the widest gap/bar is two or three times larger than the narrowest.