Toner is a dry powdery substance used in laser printers and photocopiers which forms the text and images on the printed paper. In its early form it was simply carbon powder. In order to improve the quality of the printout the individual carbon particles were blended in a polymer. The polymer particles can be melted by the heat of the fuser, causing it to bind to the fibers in the paper. The exact polymer varies by manufacturer but might be a Styrene Acrylate Copolymer or a Polyester Resin. Typically, you can print thousands of pages with a single cartridge.