Many regular folk believe that the Folk live in the burial mounds (called sidhe in the old tongue), but in reality, they are the closest thing to humanity from the Otherworld. They can cross over at any of the thin places, whether burial mounds or faery rings. The Sidhe Folk are not uniform, with many different folk that have different abilities, appearances, and lives. This often leads to mortal scholars attempting to classify them, despite their desire to resist categorization.
These scholars tend to refer to them as the Seelie or Unseelie (the Good and Bad Neighbours respectively). Despite this, these euphemisms often get those sucked into the orbit of the Folk into danger due to their belief that the Seelie are trustworthy or the Unseelie are automatically dangerous.. The Seelie and Unseelie are often bound by etiquettes from the other world, and expect to be treated with respect based on these customs, without always explaining it. The Seelie are rarely rude enough to not point out breaches of etiquette the first time they occur, but may be enraged or disappointed by repeated breaches (the number three seems to be important).
Most people's relationship to the Folk is through small, strange, interactions. Many of the Seelie will show up and attempt to help mortals and become part of their communities, doing chores or tasks around a town or homestead. They will often ask for payment in the form of coins from the eyes of dead men, bowls of milk with blood, kisses, or promises/favours, which can cause fear and friction with their community. The Unseelie will usually do the opposite, causing trouble in a community and then asking for payment to go away, this often ends in violence, but rarely with the violence going badly for the Unseelie Folk.
Some other scholars have notice that the Sidhe Folk talk about mortals when they are discussing this world, and therefore have pressed the Elves on the topic. The Elves have admitted that the Folk and the Elves have known each other since the dawn of reality. When this world was young, the Folk and the Elves would meet in the mushroom rings and feast. As time went on - the building of the sidhe brought great Grief to the Elves, and as the Folk became to be known as the more far away Sidhe Folk, the elves found their ancient friends to be reminders of the tragedy of the Aging of the World. This means that both the Elves and the Folk lament their ancient friendship, the Folk wishing to rekindle it, and the Elves seeing it primarily as a source of Grief.
Geas: The Sidhe Folk have a strange relationship with death and with pain. Ancient contracts allow the Sidhe Folk to ignore the pain of the real world. When they receive treatment, they may take on a Geas instead of undergoing recovery. Once the wound is treated, the Folk may choose to apply the geas, and the wound is immediately removed. The Folk cannot retroactively choose to take a geas, they must choose right after treatment. The Geas lasts as long as the recovery would have taken, except that Geas taken to recover from Mortal wounds never end.
The Geas is generally a small taboo or ritual that must be performed, or a ban which must be obeyed. For example: Never go without a sword, never say your name, always leave milk out for others, always offer food to guests, never refuse a challenge.
If the Geas is ever broken or disobeyed, the original wound comes back at full force.
Gifted: The Sidhe Folk are Gifted with Practical Magic, all of them have magic related to part of the world. The type of magic that they do tends to state their place in the court.
The brownies and similar folk tend to be Gifted for Peasant skills. The Knights of the Folk often are Gifted for Martial or Miliary skills (rarely both, only heroes are Gifted for both). The most coveted of the Folk are those who are Gifted in Artisan or Craftsman skills, those bless a community for sometimes centuries if properly paid. The most common folk will be imbued with Artist or Musical skills. It's possible there are Folk imbued with other types of magic, but those are generally unique and strange.
Spiritual Entity: The Sidhe Folk are not ghostly spiritual entities, but they are still creatures of some sort of beyond. This means that anything which can effect creatures with Spirit Nature can also effect the Folk. This means that they can be banished to the otherworld by a priest or kept out via the Fortress Gate, but do not require a Spirit Weapon to harm (as examples).
A Burning Wheel Monster Every Week