Hey! People mix and match the terms pretty frequently. I know I do.
As I understand it, RBA is an acronym for ReBuildable Atomizer. To me this could apply to any Rebuildable Atomizer, from bottom feeders, drippers, tank systems etc...
RDA is an acronym for Rebuildable Dripping Atomizer.
I think of mine as RBA's - they are basically the same as dripping atomizers except there's a hole in the bottom that allows juice that is fed from the bottom to feed the coil / wick and drain from the atomizer base. The builds for drippers and bottom fed Atomizers are nearly identical except many dripping fans use longer wick.
Here's a video to describe some RBA basics:
http://youtu.be/OzKAz7gojEA
Here's another one where I demonstrate how to build and wick a micro coil:
http://youtu.be/_XjH8lFurPk