Here is a broken-down cartotank showing the pieces:
https://img.fasttechcdn.com/152/1522704/1522704-1.jpg
From left to right, tube, drip-tip, top-cap bottom-cap cartomizer.
The cartomizer goes in the center of the tube (through the holes in the end caps.) The drip-tip goes into the top of the cartomizer.
Not much more to it than that. If you know how to use/diagnose a carto, the carto-tanks are the same except you also need to make sure the right amount of liquid is going into the carto (which depends on hole size, thickness of your liquid, and how frequently you vape).
A carto-tank will give very similar performance to a cartomizer, except that it feeds liquid continually into the carto, so it doesn't run dry.
The cartomizers are replaceable/discardble. They are like regular cartos, except they have holes punched in the sides at the bottom for the liquid to flow from the tank into the carto.
Have you ever tried just a cartomizer? If you like them, you will probably like carto-tanks more. If you don't you probably won't like carto-tanks. The ones I've tried tend to have a tight draw, tighter than I like. And as for flavor, I have some simple BCC's that I like better. However there are a lot of carto-tank devotees that swear by them.
I wouldn't call them the "holy grail" of flavor, though, in any case.
ETA: This is what it looks like when assembled. Also note this is not an IBTanked, but fundamentally the carto-tanks all work on the same basic principle.
https://img.fasttechcdn.com/152/1522704/1522704-4.jpg