The Fae Realm and the Human Realm are separated. The Fae Realm - also called The Underground, Underhill, Faerie, and other names - can only be entered in certain, sometimes shifting, spots, such as faerie rings, faerie mounds, and natural tree doors. Fae may only enter the Human Realm through these places, or through being called into the human realm by Name or Title (if they are very powerful) by a human. While in the Human Realm all Faeries' power is diminished.
A Human can only enter Faerie if they want to, or if someone who Has their Name or a Claim on them forces them to.

Fae

Fae are immortal, can own territory by Naming it or being given its Name, gain power (in general and over people) by Having their Names, and cannot lie - they can be misleading, but can't knowingly tell falsehoods without immediate pain and dissociation/fading - that is, they stop being able to take actions for a certain amount of time. That being the case, all Fae tell the truth, if only from a certain point of view, and most expect others to as well.
Fae are bound to their word. If they make a promise or a deal they have to keep it.

Changelings

Fae sometimes send important children to grow up in the human world, often to protect them from those who will attempt to trick them into giving up their Name. Changelings are effectively half-human while growing up in the human realm. They can lie without pain and fading, but will generally find this uncomfortable. They are unable to use their claims on people to transform them, though they can command them if they know their Name.
Changelings must make a choice between Human or Fae at some point in their lives. Before they turn back into Fae they will not know their own Name.

Humans
Humans are mortal and live in a separate realm from the Fae. They can make contracts with Fae, and they can Have Names - in fact many humans have a lot of Names given that they can't do anything with other humans' Names but but give or trade them away. Humans cannot transform fae, only command them, if they know their Name.

Names

Every person has a Name. Names give Fae power over the person or place they belong to. If they Have someone or something's Name they have the power to command and transform them or it. It's possible to resist transformation by insisting strongly enough that you remain unchanged, or by twisting the order to change to affect you in some way the one who made it didn't intend (assuming you Have your own Name, anyway). The more powerful - which is determined by their age, amount of territory, and the number of Names they Have - a Fae is, the harder it is to resist their commands. A human who knows themself very well and has a strong enough will to remain who and what they are could resist if not no-sell even the most powerful ones, though.

Giving someone your - or someone else's - Name requires that you do so willingly - even if you don't know that's what you are doing. The ignorant and/or unwary can do it without knowing, but if a person is unwilling the one told will know your name, but will not Have it. If someone who knows but doesn't Have your Name tells it to someone else, they do not Have it either.

Fae are born with a True Name, which they and no one else know - excepting the first Fae, who were made in such a way that their creator knew their Names. Fae children are free - they own themselves until such a time as they willingly tell their Name to another. Fae rulers will usually allow members of their courts to live there without telling them their Names until they reach adulthood, at which point they either become part of the court in full by telling them or they leave.

Human parents Name their children - it's their first, full, formal name - ie Elizabeth, Alexander, Michael, Anne, so long as it isn't a nickname - so children who are adopted and renamed likely won't even know theirs. Humans can't do anything with human Names but give them away and negotiate for them.

It is possible to contest territory by knowing the Name of a place and stating that it is yours, or under your power - if you believe this strongly enough, and convince others to believe it, you can slowly take it over. Convince the other owner it's yours and it is.

Sharing, Giving, Taking, and Asking for physical intimacy

Verbal Contracts are held up by magic in the Faerie Realm.

If a Faerie asks for a kiss, always ask 'what will you give me in return. This turns it into a deal. If you don't ask this, you are Giving them a kiss, which gives them a claim on your body - this makes it possible for them to transform you but not to command you. This applies to all forms of physical intimacy, though hugs and most dances give an extremely ephemeral claim.

Sharing a kiss, meanwhile, is an equal transaction. People who don't know each others names will state that they are Sharing sex, a kiss, a dance. If both sides don't state this then one could easily decide that actually they are accepting a kiss (or whichever), which would give them a claim.

Taking a kiss (or any physically intimate thing) from a person is the same as Giving it to them, in the magic's eyes. If you kiss someone who doesn't want it, they still get a claim. If you rape someone, they get a claim.

Food

If you eat food from the Fae realm, Faerie gains a claim on you. A human who does not eat faerie food will come out of Faerie in the same place and at the same time as they came in. A human who does, leaves these factors up to the whims of the realm.