As Rosa said - it is like a big
juice cart with a built-in atomizer and it screws on the battery. There are mainly two kinds - standard stuffing filled ones and reservoir types.
The stuffing-filled ones with soft-caps are easiest to use and easiest to top off with
juice when needed - just remove the carto from the battery, pop out the mouth-end cap and drip some juice into the stuffing (then clean up any drips out the threaded end, and I puff into the threaded end to clear the air channel if it has a defined center air channel). On a new fill of a blank carto - fill it totally then I rotate and let it lay on different sides to make sure every bit of stuffing is damp so it won't get singed. Then keep it damp and you can probably get 4-7 days use out of one (and I get more than that out of Boge soft-caps for 510). Run them dry and you could singe the stuffing and ruin the taste. Some people save good ones and are successful with cleaning and drying them for reuse.
The reservoir types each need whatever kind of syringe they specify to fill them. Some (mainly the clear ones) are rather new and sort of a work-in-progress and can fall apart.
The benefit of cartos is they usually hold twice as much juice as a cart and need to be topped off much less often. A filled carto does not last as long as a pack of cigs like some rip-off sites claim. But they work pretty well and lots of people like them. Another benefit is you can totally change flavors by just switching cartos.
Like atomizers, cartomizers come in standard resistance and LR (low resistance) and the LR ones are just for the 650mah+ fat-battery models which have strong enough electronics to handle the increased current draw of low resistance accessories.