An RDA is a Re-buildable Drip Atomiser (often called a Dripper) - simply meaning there is no tank (container) for juice - you just drip the juice onto the coil / cotton and vape away. There is typically a small well for the juice but these devices are very low capacity. People tend to use them as it's easy to change flavours and usually the flavour is better than you'd get from a tank.
The Goon, pictured, is a very popular RDA. The gap lifts off revealing the deck in which you simply add coils and cotton.
Usually used by more experienced vapers, an RDA is very handy to have when testing juices.
At Vaperscape we often use them to enable customers to try the various juices we have. Obviously we also sell them - typically sticking to better quality items as clones and cheaper RDAs can have issues with soft screws, poorly fitting caps and so on.