Webopedia can explain it as below,
"A type of printer that utilizes a laser beam to produce an image on a drum. The light of the laser alters the electrical charge on the drum wherever it hits. The drum is then rolled through a reservoir of toner, which is picked up by the charged portions of the drum. Finally, the toner is transferred to the paper through a combination of heat and pressure. This is also the way copy machines work."
So, the drum is what holds the image itself.. the toner cartridges hold the black and color toners that are transferred to the drum..
You will probably go through more toner than you will drums !
Still confusing like me? Ok, Most laser printer's drum and toner is one part, and others is assemble, install and sell separately, like Brother HL-2170W. And each printer only has one drum, but maybe has more toners.
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