A printer's drum unit takes the toner inside a printer and transfers it to the paper during the printing process. This differs from a toner cartridge, which creates text and graphics by fusing a dust-like powder to the paper during the printing process.
The process starts when the printer receives a print command and then the drum unit's photo-sensitive roller attracts the toner. The toner goes from the roller to the paper with the help of some heat. When a drum unit starts to go bad, most printers tell users by showing some type of error message on the printer.