RTA: Rebuildable tank atomizer. It comes with a tank to hold e-liquid. Some tanks are designed to take pre-made coil heads only and some are designed with a deck (RDTA-Rebuildable dripping tank atomizer) so you can put your own coils in and wick,but still have the advantage of being able to fill a tank with e-liquid.
RDA: Rebuildable dripping atomizer. No tank installed in the atomizer. Comes with a deck for you to attach your own coils. You can drip juice on the wick via the drip tip. You will need to re-drip after a few draws. Each type of receptacle has its own advantages. Here at Vapourmate we recommend those new to vaping go for an RTA first and if required or wanted you can explore other options later on.