A cartridge is a small, sealed plastic reservoir that holds fountain pen ink. Many pen brands use their own proprietary style, but there is one common style called Standard International, where many pen makers use the same type of cartridge. The benefit of cartridges is the convenience, however, they are not nearly as economical as bottled pen ink and the color range is usually incredibly limited.
A converter is a cartridge-shaped reservoir that allows you to use bottled pen ink when a cartridge would otherwise be your only option. Pens that have this option are usually referred to as cartridge/converter or C/C.
This table serves as a general guideline for which brands of cartridge/converter pens accept proprietary cartridges/converters, and which can use the standard international size (cartridge generally implies short). Please refer to each specific pen model for any exceptions to the below guidelines:
Which Cartridges are considered "Standard International"?