It's caused by slightly misaligned dies. Instead of the die faces being even with each other, they're at a slight angle. Does the look of the coin make sense now, with this in mind? Also note how you basically have no rim on the one side, and an extra high one on the other...they call that high side "finning" of the rim, I guess because it looks like a tail fin on the old cars.
As for value, I don't think it's extreme enough of an "error" for any premium, but I'd hang on to it as a curiosity since it only cost 5c.
Very common among Roosevelt dimes so I thought. I had so many I started to put them back into circulation. I go to coin shows all the time but seldom check out error coin tables. Some time ago I looked and there was those stupid coins with the enlarged rims selling for $1 to $10 each. Yes, I'd save that one if I were you.