Honestly, we don't really know what they are. There's still so much about them that is unclear. We don't really know where they came from, or how they survive as they do.

What we do know is that they seem to exist for the single-minded purpose of eating humans, though it is not really understood why they do this. They don't have digestive organs. They seem to have some type of rudimentary stomach, but all they do is vomit up the humans they have eaten when they become too full. They don't seem to need to eat us for nourishment, and yet they do.

Maybe if we understood them better, we could deal with them more easily...