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: