An inkjet cartridge is a replaceable component of an inkjet printer that contains the ink that is spread on paper during printing and sometimes the print-head itself. Each inkjet cartridge contains one or more partitioned ink tanks (reservoirs), electronic contacts and a chip that communicates with the printer.

Inside each partition of the inkjet cartridge tank (reservoir), is a tiny metal plate. In response to a signal given by the inkjet printer, a tiny current flows through the metal causing it to warm up. The ink immediately surrounding the heated plate is vaporised. As a consequence, the total volume of the ink exceeds that of the inkjet cartridge. An ink droplet falls out of the cartridge onto the paper. This process takes place in a matter of milliseconds which equals to near 6,000 times per second.

Usually, two separate inkjet cartridges are inserted into an inkjet printer: one containing black ink and one with each of the three primary colors (cyan, magenta and yellow). Alternatively, each primary color may have a dedicated inkjet cartridge. Some inkjet cartridges are specifically designed for printing photographs. All printer suppliers produce their own type of inkjet cartridges. Inkjet cartridges for different printers may be incompatible - either physically or electrically. Since replacement inkjet cartridges from the original manufacturer of the printer are often very expensive, some other manufacturers produce "compatible" inkjet cartridges as cheaper alternatives. Some inkjet cartridges have incorporated the printer head (most Hewlett Packard inkjet printers use this system). Usually, they are more expensive, but the printers are cheaper. Others don't include the print head, but they are more economical and the inkjet printers are more expensive (for example, most Epson inkjet printers).

Typically, inkjet cartridges are very expensive compared to the printer itself. Many people, therefore, use compatible inkjet cartridges (also called “generic” - those made by a company other than the OEM printer manufacturer) that give comparable quality, but with considerable savings. Another alternative involves modifications that allow the use of continuous ink systems that use external ink tanks. Some people choose to use inexpensive ink, knowing that, in the long term, it may damage the printer. However, the cost savings of 4 or 5 refilled inkjet cartridges may well be sufficient to pay for a new inkjet printer. If the printer lasts more than that, they have obtained the same result (economically speaking) at a lower cost than using original supplies.

Consumers are often surprised at the price of replacing their inkjet cartridges, especially when compared with that of purchasing a brand new inkjet printer. The major printer manufacturers, Hewlett Packard, Lexmark, Dell, Canon, Epson and Brother, often break even or lose money selling printers and expect to recoup their losses by selling inkjet cartridges over the life span of the printer. Since much of the printer manufacturers' profits are made up of inkjet cartridges and toner sales, these companies have taken various actions to limit the options of consumers in using aftermarket inkjet cartridges.

However, many consumers have purchased remanufactured cartridges from alternative companies to save money over buying new inkjet cartridges. This is a much better use of resources, much cheaper, and a whole industry has grown up around this idea. Remanufacturing of inkjet cartridges and other printer supplies also reduces the amount of plastic that goes in the landfill and keeps the Earth a cleaner place.