Humans, plants and animals are physical. Humans are physical AND spiritual. Angels and demons are spiritual only. While there has been documentation that they can take a physical form, that isn't their "basic" form. So yes, they are spirits, just as we are spirits "trapped" in a shell. Satan was an angel, specifically an arch-angel. He was cast out of heaven for trying to become greater than God and leading a rebellion. It's been said that was because of us, and how God favored us over the angels. Since I've never seen Biblical verses on that firsthand, I'd suggest it was probably more for pride, but we are most definitely his goal since we are loved so much by God.