As DCB said, the clearomizer is just a Carto without the stuffing, it generally works without a lot of hassle (not firing, leaking and such) and once it get's funky it's thrown away unless you want to try and re-wick it.
A tank uses a carto and hold a lot of juice, the carto can be pretty much any brand/resistance you want as long as it fits the tank design (some carto's have a larger diameter than others and can't be used in some tanks). Once the carto get's nasty you drain your tank or vape until empty then take it apart and throw the carto away and replace with a fresh one that is punched or slotted.
Some tanks are a royal pain while others are a joy to use.
A year ago we had Cart's, Carto's and Atty's, now we have DCA's, Tanks, Bottom Feeders, Clearomizers, XL Caromizers, Blow your mind Atty's and any number of new idea's that are coming out almost daily.