The Fae Realm is mostly vaguely medievalish fantasy. Despite the name, the fae realm is inhabited by many magickal species other than the Fae, it's simply that Fae have the most contact with the mortal realm, as well as having expansionist empires that span multiple regions. It is often called other things by other species, and by those in areas not part of faery empires. Overall, the fae realm is fairly low-tech, as modern technology simply doesn't work. Purely mechanical things generally work, but electricity rarely does.
The fae realm is divided into several regions, each of which are mainly one biome. The central and largest land region, the Woodlands, is mostly forest and gentle hills, and it's where most fae live. The actual largest region is of course the Seas, which encompasses the ocean. Other regions include: the Ashen Lands, a volcanic, stony shore region which also encompasses several island chains. The Darklands, which are unnaturally dark, somewhere that the sun refuses to shine. It's the source of many rare materials, and home to some strange, dangerous, and rarely seen magickal creatures. Less important regions include, but are not necessarily limited to, the Grasslands (plains, savannah), the Highlands (mountainous), the Wetlands (swamp, general areas with lots of water), and the Dust Lands (desert, other dry biomes). The fae realm isn't a spherical planet, it's simply a flat plane.
The mortal realm exists alongside the fae realm. It's roughly equivalent to modern-day Earth, though it's been stagnant technologically for a while. It's possible for humans who get lost in forests in the mortal realm or who step into faery circles to fall into the fae realm. Fae are generally at least heard of, but are rarely seen. Some fae choose to live in the mortal realm, generally under a human disguise. Fae that look significantly non-human are exceedingly rare in the mortal realm, but not unheard of. Humans generally are unaware of the particulars of magick, and the fae tend not to wish to explain it.
Vania is the largest country in the fae realm, spanning nearly half of the Woodlands, and extending into a large part of the Seas, as well as several other regions. It's ruled primarily by a queen, with a council of nobles making most of the lower-stakes decisions. The demonym of Vania is Vanii, and its royal family is the Vanian family, of the winter court. Those with Vanian blood actually have an ancestral contract, which allows them to switch between a feminine and a masculine form, though they've essentially been sold out to an unknown shapeshifting entity for this. Thankfully, this entity hasn't been seen in thousands of years. Sea-dwelling Vanii actually mostly rule themselves, and have been fighting for independence, though a war hasn't quite broken out. Of course, Vania doesn't recognize their independence at all. In mainland Vanii culture, at least that of the regions closest to the capital, it's considered rude to show the tips of your ears, so most Vanii fae wear jewelry that covers the tips of their ears.
The Minn Kingdom is a sea-dwelling, mostly underwater (unrecognized) kingdom in Vanii territory. Despite the name, it's actually mostly democratic, though they have a president-like role that approves most, if not all, laws. This role is actually usually inherited by blood, though due to a contract, it's currently out of the hands of the Minu family, the Minn Kingdom's "royal family". The Minu family is of the summer court, and are almost entirely sea-dwelling or amphibious fae. It's mostly on the shore, and has some abovewater parts.
It's currently ruled by Keimon Errata, an agent sent by the Vanii queen to stop war from breaking out through any means possible. He obtained his position by posing as a trusted advisor to the previous ruler, and convincing the previous ruler to hand the position over to him via contract when they died rather than to the one next in line, who they didn't trust fully. He then, of course, proceeded to kill the previous ruler to become the current one. He's generally made himself a problem, forcing the Minu family to keep their children seadwelling, or drown them. He also prevents pretty much anything from happening, denying just about everything that comes across his desk.
Fae are some of the main inhabitants of the faery realm. They look similar to humans, though they have pointy ears, and can have unnaturally colored eyes and hair. They can't lie, and are burned by iron. They're also immortal. Faeries are generally either of the winter court or the summer court. Fae of the winter court have silver blood, while those of the summer court have golden blood, and fae blood is somewhat transparent and watery. Half-bloods are possible, but rare, as marriage between fae of different courts is illegal in most countries. Half-blood fae are often banished to the mortal realm. Wounds are quite difficult for fae to heal naturally, though they usually don't have a lot of negative effect if not inflicted with an iron weapon. Most wounds are healed using magick, though there's also magick that makes it more difficult to heal wounds using magick. Fey of the winter court are often thought of as trickier and more deceptive, while those of the summer court are thought of as nicer and more polite.
A lot of different magickal creatures exist; too many to detail here. They range from more animalistic creatures to fully sapient people with their own societies. Fae innately have the ability to communicate with more animalistic creatures for the purpose of making contracts. The most common magickal creatures are simple elemental spirits taking the form of an animal. They usually have neutral opinions of fae if they haven't been personally wronged by one. Sapient magickal creatures often dislike fae due to their expansionism (and fae often see other magickal beings as lesser). In general, if you can think of a fictional creature, it probably lives in the fae realm.
Fae have the ability to create contracts with magickal creatures, generally granting them some of the powers of the being contracted with, as well as (usually) modifying the fae's appearance/body. For example, a faery contracted to a siren might gain a beautiful voice, able to supernaturally lure people, and the ability to breathe underwater, but also have their legs replaced with a mermaid tail. Some contracts can even outright make one into a magickal creature, though they almost always force one to forfeit any sort of faery magick and status amongst the fae. Other sorts of powerful contracts generally will take your true name or something else abstract (such as memories), require you to serve the being for a period, take an ability, etc.. Weaker contracts sometimes simply have a time limit, or require you to provide something to the being. More animalistic creatures sometimes simply ask for something frivolous or simple (like a basket of berries, or to see the mortal realm or some aspect of fae culture), or to be taken care of. Weaker contracts, like those with elemental spirits, usually don't modify the contractee's appearance at all. Of course, the more powerful the contract, the harsher the price. Some fae choose to contract with specific creatures based solely on how it will modify their appearance. Some fae families choose to all contract with the same creature. The changes to your body from contracts is usually both gradual and painful. Fae can also make contracts with mortals, fae, and other magickal creatures to grant them some of their (the fae's) abilities.
All living beings have a true name, though they don't necessarily know theirs. Fae are generally aware of their own true names, though many other creatures are not. True names are literal names, and have two parts, hyphenated. These parts are frequently inherited to some extent, though they're nowhere near exact, they simply frequently resemble the true names of your parents. Having someone's true name grants you some power over them, and the longer you have the name, the greater the power. Even non-true names have power, at least over Fae, though this power is significantly less, and doesn't increase over time.