An ink cartridge is the part that will contain the ink, while a printhead is the part in an inkjet printer that will spray the ink onto the paper.
Please note that, for a good portion of the ink cartridges on the market, these two are combined into one (the ink cartridge will also be able to spray the ink like a printhead). If you are not sure if that's the case with your inks, please confirm with your printer's manual or reach out to our customer service.
Each inkjet printer brand contains its own type of ink cartridges, which means that the cartridges are not compatible across brands. Certain printers contain ink cartridges that combine three colours -- magenta, yellow and cyan -- into one cartridge. Other printers contain each colour in its own cartridge. On certain printers, if one cartridge is out of ink, the printer will not print.
Every inkjet printer contains at least one printhead that applies ink onto paper and forms a document. Some printers contain two printheads, one for black and one for colour ink, whereas other printers may contain a separate printhead for each ink cartridge. All printheads must be in working order for the printer to print.