Fear is an emotion induced throu percieved danger or threat (whether it is real or not does not appear to matter). Fear causes physiological and behavioural changes such as the urge to flee or hide, or even freezing in the spot.
Fear may occur in response to a certain stimulus occurring in the present, or in anticipation or expectation of a future threat perceived as a risk to oneself.
Fear is modulated by the process of cognition and learning. Thus fear is judged as rational or appropriate and irrational or inappropriate. An irrational fear is called a phobia.