Theres like 500 of them out there.
I'd swing by the Modding forum and read up there a little bit, theres a bunch of scattered info there. I'll try to give you an extremely summarized version of information, this is in no way all inclusive or even 100% accurate.
1. What are they...
RBAs or RBATs or RATs etc, are rebuildable atomizers. They come in different formats. The "Genesis" style is usually characterized by having 2 chambers. One that acts like a
juice tank and another that has a + and - post to connect your coil (more on coils in a sec) a wick runs from the coil chamber to the juice chamber and you have happiness (if you get it to work). Other types are drip types, CE2 'style' and a variety of others. Genesis style RBAs are the most popular in general, though everyone has their own tastes.
2. Where can I get them!?
A variety of sources. The most readily available RBAs are the DID and DID clones, a quick search should find you plenty of info on them. They're relatively inexpensive compared to some of the more fancy models like the Zenesis but you get what you pay for there.
3. F*ing magnets, how do they work? Err, I mean, RBAs.
All RBAs have a coil and a wick in different configurations. For the coil you use NiChrome or Kanthal "resistance wire" you make a coil out of the wire of choice and connect it to the positive and negative posts in your RBA, wick configuration is dependant on your setup. For Genesis style people tend to lean towards stainless steel mesh, 400x400 mesh is the most common these days but a lot has been said abotu 500x500 (500 holes per inch, 400 holes per inch, etc) You roll it up, oxidize it via flame, insert it in your juice and wrap your coil around it. Again, this is a horribly simplified version of what you should read up on.
4. Should I get one?
Yes, if only a cheap one to play around with. They are an 'advanced'
vaping product in that it takes a lot more fiddling than just squirting some juice in a carto and honkin away on it. You'll want to read, watch videos, etc or you'll end up stabbing the nearest organism with whatever you have handy due to frustration. It's a good experience when you get it, and once you get it, you 'Get it' and can recreate that experience with less and less frustration every time.
Theres A LOT of info on these forums and elsewhere on the interwebs, do a google for "Genesis style atomizer" then a youtube. You'll have more videos and webpages than you can shake a provari at. Enjoy.