An ink cartridge is a replaceable component of an ink jet printer that contains the ink (and sometimes the print-head itself) that is spread on paper during printing.
How ink cartridges work
Thermal Inkjets: Most consumer inkjet printers, such as Canon, HP, and Lexmark (but not Epson printers) use a thermal inkjet; inside each partition of the ink reservoir is a heating element with a tiny metal plate. In response to a signal given by the printer, a tiny current flows through the metal making it warm up, and the ink immediately surrounding the heated plate is vapourised into a tiny air bubble inside the nozzle. As a consequence, the total volume of the ink exceeds that of the nozzle. An ink droplet is forced out of the cartridge nozzle onto the paper. This process takes a matter of milliseconds.
The printing depends 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 soft paper towel.
The ink also acts as a coolant to protect the metal-plate heating elements: when the ink supply is beginning to run out, and printing is attempted, the heating elements in thermal cartridges often burn out, permanently damaging 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.
Piezoelectric Inkjets: All Epson printers use a piezoelectric crystal in each nozzle instead of a heating element. When current 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.
Ink Cartridge Variants
Typically, two separate cartridges are inserted into a printer: one containing black ink and one with each of the three primary colors. Alternatively, each primary color may have a dedicated cartridge. Some cartridges are specifically designed for printing photographs.
All printer suppliers produce their own type of ink cartridges. Cartridges for different printers may be incompatible - either physically or electrically.
Since replacement cartridges from the original manufacturer of the printer are often expensive, some other 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, better, or inferior quality.