RDA stands for Rebuildable Dripping Atomizer. An RBA is, simply put, a tank without a reservoir. The deck contains 2 or more connections (posts) to attach your coil (s) to which you then put your wick. Over that you place a hood (screw or sliding system) with a connection for a mouthpiece in that cap.
RDAs are designed to e-liquid directly on the wick to drip after which you can take a few shots. It is generally assumed that RDAs give a much more intense and more pure taste experience than all other existing systems with a reservoir because of this drip system.