Inkjet ink comes in cartridges and individual ink tanks. Cartridges have more than one color in a single unit.
Some printers use individual ink tanks, and are said to save you money in the long run. Rather than replacing an entire cartridge, you need to replace only the color that runs out.
Many photo printers use six, seven or eight individual ink tanks. A combination of extra colors and/or varying shades of black means that prints will have more true-to-life color.
Ink types
Dye based inks provide excellent color but are not light fast or water resistant. Dry slower than pigment-based inks.
Pigmented inks are fast drying and offer better resistance against fading than dye based inks.
Archival inks resist fading. Some manufactures claim their archival inks will produce color prints which last over 100 years. Archival paper should be used for printing with archival inks.
To refill or not to refill
Debate is strong as to whether cartridges should be refilled to save money.
Current research seems to indicate it's best to use ink made by the company who made your printer. Testing reveals that these prints last longer (permanence). Some say they refill ink cartridges without problems. Others report that refilling clogged or damaged their printer.
If you own a new ink jet printer and decide to refill the cartridges, contact your manufacturer to see if doing do will void any remaining warranty.