Fear is a distressing emotion caused by impending danger, evil, pain, etc, especially dread to meet or experience the danger.

Psychologists such as John B. Watson and Paul Ekman have argued that fear, along with a few other basic emotions such as joy and anger, is innate in all human beings. Fear is a defensive, survival advantage, and may have evolved in a variety of organisms. It is usually a response to a particular stimulus. For example, a person may see a spider and experience fear. Fear serves as motivation to escape to safety.

An example of this may be something dangerous and spontaneous, during this situation the blood goes to big muscles (like legs) and adrenaline is pumped out to the muscles allowing the person to run faster. In addition, the body freezes up just an instant allowing the brain to decide if another reaction would be better (like hiding). In the brain, hormones are released centering the attention on the threat always looking for the most accurate reaction.

Fears may be a factor within a larger social network, where in personal fears are synergetically compounded as mass hysteria.

* Paranoia is a term used to describe a psychosis of fear, described as a heightened perception of being persecuted, false or otherwise. This degree of fear often indicates that one has changed their normal behavior in radical ways, and may have become extremely compulsive. Sometimes, the result of extreme paranoia is a phobia.

* Distrust in the context of interpersonal fear, is sometimes explained as the inward feeling of caution, usually focused towards a person, representing an unwillingness to trust in someone else. Distrust is not a lack of faith or belief in someone, but a feeling of warning towards someone or something questionable or unknown. For example, one may "distrust" a stranger who acts in a way that is perceived as "odd." Likewise one may "distrust" the safety of a rusty old bridge across a 100 ft drop.

* Terror refers to a pronounced state of fear - which usually occurs before the state of horror - when someone becomes overwhelmed with a sense of immediate danger. Also, it can be caused by perceiving the (possibly extreme) phobia. As a consequence, terror overwhelms the person to the point of making irrational choices and non-typical behavior.

Fear can also affect the subconcious and unconcious mind, most notably through nightmares.

Source : http://en.wikipedia.org