The actual encasement, which holds the ink or toner. Some cartridges function as an "ink tanks", while others actually contain a print-head assembly that disperses the ink onto the paper.
An ink cartridge is a replaceable / refillable component of an ink-jet printer that contains the ink (and sometimes the print-head itself) that is spread on paper during printing.
A toner cartridge is a replaceable / refillable component of a laser printer that contains the toner (powder) that is spread on paper during printing. Toner cartridges are made of a core (cartridge shell), as well as many components such as wiper and doctor blades, OPC drums, developer rollers and more.
A drum cartridge is a replaceable / refurbisheable component of a laser printer that contains the OPC drum.
See also: