They are indeed a grape (looks like a green grape) & It's a cross-breeding thing...has to do w/ how the flower is pollinated. I've heard they cross a concord w/ another type of grape. Whatever they do, it tastes just like cotton candy and I ate a whole bag yesterday!
Yeah, I probably shouldn't have, but they are only here for a few weeks & then they're gone (small crops).
I've attached a link here from an LA Times story:
The article says the grape growers are coming up w/ new varieties to compete with other snacky-type foods.
I bought them at a Gelson's in Orange Co., CA. The crop is small, so not sure where all they're available, but they are increasing acreage so there should be more in coming years.
If you come across them, try them - they're delicious!