RTA is short for "Rebuildable Tank Atomizer", a pressure differential liquid tank that surrounds and feeds the atomization chamber.
Electronic cigarette with option to add your own special dry blends, e-liquids, and others, heated within its chamber
"Mod" is short for modification. An e-cig "Mod" in its simplest form is a battery tube. Common types are 18650/18350/26650
RDA is short for "Rebuildable Dripping Atomizer", typified by a short, 510 base threads at bottom, and two posts (+/-) on its top