We experience fear on an almost daily basis to some degree and in some form. Fear is a learned response to certain situations that can keep us safe from potential harm, for example the fear of heights. A mild fear of heights would be considered quite logical given the potential danger of falling. 

However, a Phobia develops when the fear becomes irrational and interferes with our daily lives. Common Phobias include Spiders, needles, public speaking, dentists and flying.