Due to the books relying heavily on mystery and surprise, the pages would be virtually unreadable with excessive spoiler tags. Tread carefully. They're mostly divided into the warring Summer and Winter Courts, with those outside the Courts known as "wyldfae".
The Fae are ruled by a Byzantine series of laws and restrictions that govern how various members of each Court can act, but they have become masters at working within these restrictions.
For example, as a rule they Cannot Tell a Lie , but they're absolutely ruthless rules lawyers about Exact Words , and being in debt to a faerie is They range in power from the relatively harmless pixies to the greater-than-godlike Mother Summer and Mother Winter. Luckily for the Puny Earthlings , all Fae have an Achilles' Heel : iron and steel will burn them and cut through their magic instantly. Achilles' Heel : Iron and any alloy thereof most commonly steel , which is considered the equivalent of toxic waste.
Leaving iron objects in Faerie is considered a dire insult. Overall, the Fae are noted for having a wide range of weaknesses, some very specific, and some seemingly quite arbitrary. Getting around these weaknesses and limitations is what makes the fae so damned dangerous and such effective manipulators. Always Chaotic Evil : Played interestingly in regards to Winter. They're pretty much all harsh, brutal, and unforgiving owing to the nature of the power they're aligned with, but the books also plainly establish that they aren't evil in a metaphysical sense like the Fallen, Outsiders, and other demons.
And then in Cold Days it turns out Winter exists to protect the rest of reality from the much, much nastier Outsiders, which blows this trope pretty much out of the water while simultaneously giving a really good explanation for why Winter fae need to be what they are.
