An ink cartridge is a replaceable component of the ink jet printer that contains the ink and usually the print-head itself which spreads the ink on the paper during printing. For most color inkjet printers there are two cartridges docked into the inkjet printer itself, monochrome inkjet printers usually only dock one cartridge. Each of these ink cartridges contains one or more partitioned ink reservoirs, the color always has at least three holding the magenta, cyan and yellow colors(Alternatively, each primary color may have a dedicated cartridge), while black has only one reservoir; certain manufacturers also add electronic contacts and a chip that communicates with the printer.

The main consumer inkjet printers, such as Canon, HP, and Lexmark (but not including Epson printers) use a thermal inkjet; inside each partition of the ink reservoir is a heating element with a tiny metal plate or resistor. In response to a signal given by the printer, a tiny current flows through the metal or resistor making it warm up, and the ink immediately surrounding the heated plate is turned into vapor and becomes tiny air bubbles inside the nozzle. As a reaction to this event, the total volume of the ink exceeds that of the nozzle. An ink droplet is forced out of the cartridge nozzle onto the paper. The whole process takes place in a matter of milliseconds.

The printing relies on the smooth flow of ink, which can be hindered if the ink begins to dry at the print head, as can happen when an ink level becomes low; dried ink can be cleaned from a cartridge print head, by gentle rubbing with isopropyl alcohol on a swab or folded paper towel.

The ink is also a coolant to protect the metal-plate from over heating elements: when the ink supply is depleted, and printing is attempted, the heating elements in thermal cartridges often burn out causing permanent damage to the print head. When the ink first begins to run thin, the cartridge should be refilled or replaced, to avoid over-heating damage to the print-head.

All Epson printers use a piezoelectric crystal in each nozzle opposed to a heating element. When electrical current from the printer is applied, the crystal changes shape or size, forcing a droplet of ink from the nozzle. A piezoelectric inkjet allows a wider variety of inks in a much finer quality than thermal inkjets, while more economical in ink usage.

All the major printer suppliers produce their own brand of ink cartridges. Cartridges for different printers are usually incompatible with other printer makes and models - either physically or electrically.

Since OEM replacement cartridges for the printer are often expensive, some third party manufacturers produce "compatible" cartridges as inexpensive alternatives. These cartridges sometimes have more ink than the original OEM branded ink cartridges and can produce the same or better print quality.

Some people choose to use aftermarket inks. They can either re-fill their own ink cartridge, buy aftermarket remanufactured brands, or take them to a local re-filler. Consumers are often surprised at the price of replacing their printer cartridges, especially when compared with that of purchasing a brand new 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 cartridges over the life span of the printer. Since a large part of the printer manufacturers' profits are made up of ink and toner cartridge sales, some of these companies have taken various actions against aftermarket cartridges.

Another option is for the consumer to re-fill the cartridges. Instructions for most cartridges are available on the web, as well as sources who sell "bulk ink" in pints, quarts, and even gallons. This can be extremely cost effective if the consumer is a heavy user of cartridges. One pint (473 ml) is sufficient ink to fill approximately 15 to 17 large cartridges of a typical 27 ml capacity.

Many consumers choose to have their cartridges re-filled or to purchase remanufactured cartridges from third party companies to save money over buying OEM cartridges. This is much cheaper because you only need to pay for the ink and our whole industry has grown around this idea.