Fear is a feeling that everybody knows as being an entirely normal feeling. Fear usually is a reaction to a frightening situation such as a bank robbery or any other dangerous situation.
Normal fear turns into anxiety disorders or phobia when a fearful reaction occurs that does not correspond to the actual event, i.e. a situation that the majority of people would rate as not frightening at all, or if the anxiety reaction is so abnormally strong that the patient has the impression he is going to vomit, to faint, to die or to go insane.