A Ribbon Cartridge is a replaceable cartridge that is sold together with the fax ribbon. A Ribbon Roll is sold without a cartridge.
Some fax machines such as Brother Fax 560 and Brother Fax 580MC require a ribbon cartridge. Other fax machines such as Brother Fax 575 and Brother Fax 878 are designed with a cartridge which can be refilled with a ribbon roll.
Compatible ribbon cartridges exists and allows fax users to save money. Quality cheap ribbon rolls are also available (such as Brother Fax 575 ribbon) outside fax machine manufacturer stores.