RBA is a general term used for a rebuild-able atomizer (your coil, wick, and sometime a tank), and an RDA is a rebuild-able dripping (called a dripper because you regularly have to fill it by dripping liquid on it) atomizer, one without a tank, and usually a shallow juice well. There are also RTAs which is a rebuild-able tank atomizer, same thing, just has a tank for your juice so no dripping is required. Also welcome to EFC!