There are mainly two types of technologies implemented in barcode printing.
Thermal transfer printers use printheads to generate heat which melts a wax/resin substance on a ribbon, transferring ink from the ribbon to the label/tag material. This method produces images that are permanent and crisp, and is used for paper/synthetic materials.
Direct thermal printers use the heat directly on a specially coated paper material, causing a chemical reaction that turns the paper into dark color/black. Direct thermal printers are generally less expensive, but the dark color printed on the labels might fade away (making the barcode illegible) due to heat exposure, direct sunlight or chemical vapors.