In laser printers, the toner is applied to the media via a charge image from the image drum, the Photo Conductor Unit( PCU). In order for the toner powder, which is only stabilized by the charge image on the media and lies quite loosely on it, to be firmly bonded to the media, it is burned into the media.
The fuser unit, which is located at the end of the paper path just before the ejector, does the fusing. In monochromatic printers that print only black and white, the fuser unit consists of a heated pair of rollers. One roller is the loading roller. It positively charges the paper to be printed. This allows the negatively charged toner to be attracted to the image drum. The toner is heated above its melting point, liquefies and is pressed onto the paper by roller pressure.
The situation is different with color laser printers, where the fuser unit consists of four image drums whose four colors are applied to the print medium one after the other in the sequence yellow, magenta, cyan and black and then fixed with heat.