For example an HP toner cartridge holds toner and dispenses it as needed to make copies (in a copier) or to prints pages (in a printer).
The photoreceptor drum is the surface to which the toner adheres before being transferred to the paper. The drum has a long but finite useful lifetime and must be replaced occasionally. Some copiers/printers have drum cartridges which can be changed out by the user. The alternative would be to have a service person open the machine and replace the photoreceptor drum.
So, a drum cartridge allows the user to replace the photoreceptor drum and a toner cartridge allows the user to replenish toner supplies.