Fantasy is like coffee. Fluid, delicious, and occasionally a work of art. And, like coffee, fantasy comes in many different varieties. You probably think of Tolkien-esque fantasy, with lots of magic and plenty of cool new fantasy creatures. But, like Sci-Fi, fantasy comes high and low. High Fantasy is what you get when you read Lord of the Rings, or play Dungeons & Dragons. High Fantasy refers to a world where magic and other such fantasy elements are commonplace or well known. Typically, the world will be filled with cool imaginary creatures, like orcs, giants, and of course dragons.
Low fantasy, on the other hand, is the opposite. Magic and other traditional qualities of fantasy are rare, though they may be commonly believed in. Low fantasy worlds often resemble our own Medieval Era, with monsters a rarity, though Low Fantasy is not Medieval Fantasy. Most importantly, supernatural events are seen as abnormal, whereas High Fantasy worlds often consider these standard. Low Fantasy also tends towards darker fantasy, though not quite as dark as Dark Fantasy. An example of Low Fantasy is the book Duskfall by Christopher Husberg.
Now that we’re on the topic, what is Dark Fantasy? Dark Fantasy typically depicts a grim world, where magic is either dead or dark, and creatures are twisted and unnatural. Dark Fantasy worlds will usually be crawling with horrible monsters, pitting their residents against skeletons, dire rats, and other such unpleasant aberrations. These stories typically deal with bleak or nihilistic themes, and harbor little hope. A good example of this genre appears in the game Dark Souls by FromSoftware.
Another popular genre of fantasy is Urban Fantasy/Historical Fantasy. Both are pretty similar, effectively inserting fantastical elements into the present or past. The greatest challenge that comes writing this genre is worldbuilding. It’s often difficult to justify having our world turn out exactly the same with magic in the mix. I recommend looking into the movie Bright for a terrible example of this. Most worlds avoid this issue by making the magic either very rare, or, more commonly, hidden from our world. This makes it so that the author has to do very little worldbuilding, saving them a lot of work. This is also often the case in Historical Fantasy, though some authors take advantage of the widespread belief in the supernatural throughout most of history, proposing that that belief is not entirely unfounded.
These four subgenres of fantasy are only a few out of many. There are myriad types of fantasy, and I have only selected the most popular. I suggest you look into other types of fantasy, and continue reading it.