Cartridges for the C64 provide either 8K or 16K of ROM that will replace parts of the memory visible to the CPU when plugged into the expansion port. Either 8K or 16K, that is all the Commodore can see directly on a cartridge. Then there is an auto-start mechanism implemented in the Kernal that can be triggered to execute code contained in that ROM when the machine is powered on or reset. Jani features an excellent article about how to create such simple cartridges on his site which I have linked to before.
A simple ROM cartridge: the OpenC64Cart.