Hi Dr Phill ,A tank Is (generally) a resivoir, that feed the cartomizer through a hole in the outer wall of the cartomizer. They work on the basic siphon principle, when you suck air through the air passing through pulls a little of the liquid into the filler of the carto to re-wet the filler and replentish what you have just vaped away.
Ther are other tank type things Like the Vivi Nova, Kanger t-2 and the rebuildable Stardust that are an atomizer suspended in the tank with a wick that feeds the atomizer coil, these work on a similar principle except the heater is usually a replaceable part instead of being inside a cartomizer. Most of these are either 510 threaded(Vivi) or eGo threaded (T-2 & Stardust), eGo thread being the outer skirt thread rather than the standard 510 inner....you may want to consider an eGo style battery to go with either type because generally the tanks, either style, use a low resistance coil or carto...... Good Luck. Happy Vaping.