To put it very simply: Fears are Slave-drivers. Almost every Fear has some sort of cult or Servant working for them. Fears are, as perhaps part of The Great Game , very antagonistic towards humans. They actively chase after humans and kill them. 

When a human is chased by a Fear, they become what is known as a Runner. Runners are formed by the creation of a mental bond between Fear and Runner, which allows the Fear to know exactly where the Runner is at all times. The Fear will use that information, and possibly servants, to torture the Runner, sometimes mentally, other times physically. 

When the Runner breaks (though some escape, forming a Phobium), the mental link is completed, and the Fear can now control most aspects of the Servant's being. The Servant tends to develop a sort of love for their Master, and are sent on tasks such as murder, tormenting of runners, and torture. 

However, everything above is largely generalization, as Fears and how they treat or obtain servants all vary wildly. Below is a list of Servants and their corrosponding Fears: 

Archivists - Servants of The Blind Man, who call him the Scribe

Campers - Litterally EAT, posessing and controlling what ammounts to empty shells.

Dolls  - Servants of The Wooden Girl, who call her The Misteress 

Drones - Servants of The Intrusion, who call it The Swarm

Greyskins - Slaves to The Choir, who do not revere it enough to acknowledge its mastery

Judgment - Literally The Eye itself

Lillims - Servants of The Mother of Snakes, who know her as the Scaled Mother

Maenads - Servants of The Rake, who know it as The Beast

Nests  - Servants of The Convocation, knowing it as The Avian One

Oath Breakers  - Servants of The Plague Doctor, who know him as The Good Doctor