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.