The Eye, or Beholding, is obviously the most integral fear to the series. It is the fear of being watched, of having your secrets known. The Magnus Institute is under the control of Beholding, and it is fed off of the statements of those affected by other Fears

The Stranger, or "I do not know you", is the fear of strangers, the uncanny, or the unfamiliar. It sometimes manifests in almost-human monsters, things made of wax or faux skin. it feeds off of the fear of those who know that the monsters are not who they say they are

The Web, The Spider, The Mother of Puppets, is the fear that your actions are not your own. That you are not in control of yourself, being forced to dance to someone else's tune.  It is mostly manifests in the form of spiders or webs in some way.

The Spiral, or "It is not what it is" is the fear of madness, that what you precieve is not actually happening. It manifests in many ways, and they are characteristically hard to explain. The main manifestation is in the form of a Yellow Door that takes people to an unexplainable maze.

The Lonely, or "The One Alone", is rather simply, the fear of being alone, that you are the only person left in existence. It doesn't usually manifest in any sort of monster, it just steals people from time to time and isolates them, feeding off their fear. 

The Vast, sometimes The Falling Titan, is also simple. It is the fear of heights, huge things or spaces, or the deep ocean. It also doesn't often manifest in the form of monsters (at least not ones that aren't incomprehensibly enormous). It also just steals people and lets them fall forever.

The Buried, or "Too Close I Cannot Breathe" is kind of the opposite of the Vast. It is the fear of being buried, or crushed, or tight spaces. It manifests by burying people, it's not terribly complicated. It also sometimes manifests in the ocean, but the focus is more on crushing than the vast space. 

The End, Death, or The Coming End That Waits For All And Will Not Be Ignored (hell of a name, I know), is just as the name suggests, Death. It feeds off the fear of the inevitable end of all life, manifesting in Reapers playing games of chance and the Corpse Roots.

The Slaughter is the fear of senseless, unpredictable violence. It manifests in the from of war. In wild, manic violence, or calm, cold and controlled soldiers killing without care. It doesn't manifest nearly as much these days, but it was quite active during and before the World Wars. 

The Desolation, or The Lightless Flame, is similar to  The Slaughter, in that it is the fear of destruction. It's not nearly as specific though, its simply the fear of everything you love being taken away. It manifests in flames, destruction, and well, Desolation. 

The Hive, or the Filth, or The Crawling Rot, is the fear of bugs, corruption, disease, basically anything gross. It manifests in any number and type of bugs, and diseases, rot and mould, man-made or natural. It can also occcansianlly manifest or result in unhealthy and toxic relationships. 

The Dark, or the Forever Blind, is also incredibly simple. Its the fear of the dark. It manifests in monsters that have no form and cannot be seen, but kill nonetheless. Pitch black, ice cold water and "brackish" water is also a key part of its manifestation.

The last three are rather unique for different reasons. The Extinction, for example, is a new power that is in the process of emerging. It is the unique fear that mankind is going to destroy itself, and the fear of what will come after, nuclear winter, climate change, that kind of thing. It does not have many proper manifestations yet as it is new. 

The Flesh is not borne of the fear of mankind. It is the fear of the billions on animals on the planet that humans raise for slaughter. Pigs, Cows, chickens, that kind of thing. Because it's not a "human" fear, when it interacts with humans it gets strange. It manifests in endless slaughter houses and the morphing and changing of flesh. 

The Hunt, or simply "Blood" is also an animalistic fear, and it is the fear of prey being hunted.  When it manifests and interacts with humans, it often results in people becoming more animal-like, hunting and killing other humans for fun or sport.