Difference between a drum and toner cartridge?

What is the toner cartridge?
The toner cartridge contains a fine dust-like powder (toner) that fuses on to the page to create graphics and text.
When you notice faded prints or streaks or low-quality printing, it is time to replace the toner.
Example 1 – Here’s an image of a toner cartridge:

Example 2 – Here’s an image of a 4-pack of coloured toner cartridges:

What is the drum unit?
The drum unit is the component responsible for transferring the toner onto the paper.
The way it works is that when the laser printer receives a print command, the toner is attracted to the photo-sensitive roller inside the drum unit. The toner is then transferred from the roller in the drum unit to the actual paper using heat.
When the drum deteriorates with use most laser printers will display an error message on the printer interface to let you know it needs to be replaced.
Example 1 – The drum unit of a monochrome laser printer

Example 2 – The drum unit of a coloured laser printer (black, yellow, magenta and cyan)