it's fairly common in the elderly and those who party hard (it used to be called "holiday heart" because sailors on shore leave would get so shitfaced their hearts would wig out) but mine has no clear cause. a-fib usually isn't a serious condition, it just kinda feels like you're a bit winded
v-fib on the other hand is serious shit, that's when the ventricles (bottom half) that slam out the fresh blood into the body lose their marbles and just do whatever
if you are on a-fib but your Vs are working fine, your blood is still circulating (if a bit unevenly) - but when your Vs take a shit basically your circulation system goes to hell
this all gets traced back to how cardiac cells work - each cell is capable of spontaneously generating its own signal and flexing, but those weak tendencies are (supposed to be) overridden by the big signal coming down the sinoatrial nerve that makes them all work (flex) together. when the big signal gets interrupted or garbled, the cells just kinda do their own thing
here's a gif of what normal sinoatrial signal looks like:
you see the big signal come down and force everything to work together
and here's what a-fib looks like:
big signal gets gunked up or cut off - leaving the atria to just sorta fuck off and the valves just sorta flap around BUT the bottom half still pumps your shit around so you don't die, you just feel kinda weird (i described it as "my heart feels like holding a declawed cat over a bathtub full of water")
on the other hand here's what v-fib looks like:
your ventricles are having a goddamn rave and if you get this you pretty much die within a minute or two
THE MORE YOU KNOOOOW
Afib is soooo common and pretty easy to deal with now a days. Some nights I feel like everyone is in afib! Vfib is the oh sh** shock rhythm. I used to be more scared of whacky heart rhythms and especially chest pain/heart attack type stuff but there's a lot we can do now for hearts. Watching open heart surgery is so fascinating, especially valve replacements! I did a shift in the cath lab so I could practice pulling the sheaths they put in when they do angiograms/stents/ablations etc. Watching them do stents & seeing the arteries flow again smoothly is pretty cool to watch too. I take care of pacemaker pts all the time ...they can do different kinds so some are just defibrillators and others can pace as needed or all the time. People are in n out for them it feels like! 
This sort of emphasizes how shitty it is that an employer can fire you when your doctor has you admitted and then put in a bed for 5 days for your own safety, legally.
