An RBA is a rebuildable atomizer. Basically you buy some wire and some wicking material and build your own coil and wick, then fill up the tank and vape away.
An RDA is a rebuildable dripping atomizer. The same thing except that there is no tank for juice; you need to manually drip liquid onto it from time to time.
A good starter RBA would be the
Smoktech RSST, and a good RDA for a beginner would be either the
IGO-L or the IGO-W. I'd recommend clicking those links and watching PBusardo review and build them, that will get you introduced to how they work.
Also, check out the
Rebuildable Atomizer Systems forum here on ECF. Lots of great advice and tips there.
Good luck.