Tanks/Clearomizers Come in a few forms generally refered to

Tanks = A tank that takes pre made factory coils (simple to use some can provide great flavours and clouds.

RTA = Generally a tank but instead of using Pre made factory coils you have to build your own (these can be really good for flavor and clouds) another bonus of these are you can customize the way you want to vape by the coil that you build

RDA = Generally does not hold more than 1ml of E-Liquid. You build your own coils on these too.

RDTA= Like A RDA and a RDTA generally the coils sit on top of a tank that hold anything between 2-8mls of E-Liquid which then gets sucked up by the cotton. (These can be good but you would have to get wicking the cotton right or it can leak)