The Sky People are the ones who took us in. They were all born on this huge space station that crashed -- that Camp Jaha place people have been talking about lately is the original ship that people were living in until it started to snow -- and as far as I can tell, they took over Mount Weather from people who never left Earth and holed up in here instead. The people who were here were the Mountain Men, and apparently they were just a bunch of assholes. And now they're dead assholes.
The Grounders are basically everyone else that stayed on Earth. We haven't gotten a whole ton of detail, but there's some history between the nearby Grounder leaders and some of the Sky People here. They were friends, then they weren't, I guess they are again? We're not at war but we're stuck in this shitty trade deal with them that basically means we have to kiss their asses whenever they show up, but they're allowed to do whatever they want, or boom, we're at war. Not awesome. Their soldiers know English, but the rest only speak their own language. Learning's mandatory if you might come across them, so you don't insult them by accident, I guess.