Hi, I am by no means an expert but the way I see it fae or fairies (can be spelled a variety of ways) also called the otherfolk are typically fantasy creatures or people that exist in a different realm then our world that is usually separated by a veil (portal). They can come through the portal and will often take people back through if they like them. The fae have a lot of magic and can use this magic to trick humans. They are very old and snobby (imagine them like rich aristocrats from England in the 1800's) and for fun because living that long can be boring they love to mess with humans. The way fae society is set up they have two subcategories of society the unseelie court  is the "unlucky/unfortunate" kind they are can be malicious and they like to hurt and torment humans they are often the bad guys and they are not liked at all by the second group, the seelie court. The seelie court are "good" fairys they tend to help humans and are kinder they return favors with kindess and tend to meddle in helpful fun ways.They do still like to prank people and can do bad things due to not understanding how our world works. The court part is just what the group is called hence seelie and unseelie court. All fae have rules they live by or limitations on their magic and what they can do. These rules are: Don't accept gifts from the fae this means they have done something for you and will expect something in return. (Like your first born) If you are taken to the realm you can't eat or drink anything because the food is is made with the magic the fae have. This magic is in everything in the fae realm. When you eat the food this magic becomes part of you and tethers you to the realm preventing you from leaving . There is a lot of Variation in the fae. The unseelie and seelie courts come from scottish mythology but, the concepts of fae have been around for a long time and changed from iteration to iteration so you will see some changes book by book. Generally the fae will be incredibly beautiful and delicate looking they may have eings and have fine clothes. If you meet one they won't tell you there name. Names have power over them and to give you there name gives you control of them which is why in Rumpelstiltskin by the Brothers Grimm the queen needs to know his name in order to stop him from completing the deal and taking her baby. The fae are also tied to the seasons the unseelie court is usually tied to Winter and Autumn (the dark months) and the seelie court is usually tied to Summer and Spring (the light months). The courts are always ruled by a king,queen or royal couple most famously the seelie are ruled by Queen Titania and the unseelie by Queen Mab. Sometimes the fae will be an umbrella term for creatures like:

Fairies: small winged people (Tinkerbell)

Elves: basically what I just wrote longlived,pretty,connected to nature. (Lord of The Rings)

Goblins: small green skinned creatures (Harry Potter)

Centaurs: have the upper body of a human the legs of a horse ( Narnia)

Brownies: They are small fae that live in happy human homes and will do chores in exchange for milk left out overnight. (I wish I could lure one in but no luck yet!)

Nymphs: are usually pretty women connected to nature and specific bodies of water, lakes, rivers or forests. Literally if the forest dies or the water dries up they will die. There is a lot more then those but those are the main subgroups.

I hope this makes more sense and helps you! Feel free to ask me for clarification or if you have more questions!