An RDA is an atomizer. Specifically, a Rebuildable Dripping Atomizer, also known as a dripper. The function of an RDA is to route electricity from your battery through one or more coils of resistance wire, that most commonly look like little springs. Fans of RDA's will tell you that they deliver the best flavour and vapour production you’ll get out of your vape, providing you build your coils right. With an RDA, you’ll build your own coils and fill them with a wick, most commonly made out of cotton. An RDA will have no tank, meaning that you’ll need to drip your e-juice on top of the wick as you go.