The full-form of RTA is rebuildable tank atomizer, essentially with a tank which holds the e-liquid. While the capacity on the RTA tanks vary according to brands, there is one which suits every vaping style.
RDA is the abbreviation for Rebuildable Dripping Atomizer. The only differing factor of RDAs from RTAs is that they enable you to drip E-liquid directly on the e-cigarette wick. You can regularly vary flavours with RDAs.