RBAs are ReBuildable Atomizers like the Kayfuns, Russians, etc. he They have a single coil on the deck and tank to hold 4-5 ml of juice.
RDAs are Rebuildable Drip Atomizers like a Plume Veil, Magma, etc. etc. They can have many different coil configurations, some include the option for a single coil. These you drip the juice, a few drops at a time, then you get a few vapes until you drip some more juice, repeat.
The RBAs may be better for a beginner but it depends on whether you want a tank or not. RDAs are more flexible, but you must drip. RDAs are particularly good if you make your own juice, because you can just change the wick and drip a new flavor.
RDAs generally have better airflow control, better vapor production, and better flavor.
In my opinion, the new Aspire Atlantis offers the best of both worlds; a tank that holds 2 ml of juice and great flavor and vapor. And if you are not ready to get into coil building, just buy the replacement coils.
HTH