The toner cartridge is the container that holds the toner powder, which creates text or images on the page. The drum unit is an electrically charged cylinder that transfers that toner to paper. A drum unit can be incorporated with the toner cartridge or sold separately as a single unit, depending on the consumable requirements of your printer.
Laser printers and their consumables vary across printer models. These subtle differences can be a bit confusing if you are not familiar with the consumables that are specific to your printer. All laser printers require a toner cartridge and a drum unit to produce a print. However, some printers only need you to replace the toner cartridge, and others require that you regularly replace both the toner cartridge and the drum unit. Most printers use toner cartridges with the drum unit built into the cartridge. If your printer uses a toner with an incorporated drum unit, there is no need to replace the drum separately, since it is already a part of the toner cartridge. Every time you buy a new toner, you are replacing the drum. Some printers, like most Brother laser printer models, for example, use a separate toner and drum unit. In this case, the drum unit is not built into the toner, and therefore, it must be replaced, typically after the use of 3-4 toners. Your printer should inform you when it's time to replace the drum, but you can usually tell your drum is on it's last legs if you start to see black spots or lines across the page. To determine whether your printer uses a toner cartridge with an incorporated drum unit, or if your printer sees the drum unit as a separate consumable, consult your printer user manual.
Toner cartridge with an incorporated drum:
Toner cartridge and drum unit as separate consumables: