Fear is considered to be one of the most basic emotions, along with joy, anger, and grief. These emotions are universal to human beings, and are associated with specific physiological changes in the body. True fear activates the sympathetic nervous system, which controls the fight-or-flight response. Hence a person who is genuinely afraid will undergo changes in brain function, neurotransmitter activity, blood flow, and a variety of other bodily mechanisms. While these changes can be life-saving in a true emergency, they do upset the body's internal stability, and the health of a person who is subject to fear on a regular basis may suffer over time.
As far as what things make you feel fear, there are some sensations that provoke fear in almost everyone, like the fear of being injured or the fear of dying or the fear of attack or threat. Beyond that, fears are pretty individualized; what makes you fearful may not bother others as much. Some of our fears may be based in childhood trauma; it's also been suggested that the things you fear may be symbolic of subconscious issues or archetypes.
I think what makes fear real more than anything else is the very real impact and effect that it has on us. Even though so often fears can be based on things that are not real, such as the fear of being attacked if you go down a dark alley, we cannot deny that this fear has a very definite impact on us in terms of making us more anxious and making our heart beat and breathing speed up. In the same way, if we take a classic example of a child at night being scared that monsters under his or her bed are going to come out and gobble him or her up, we can see the similar real effects of unreal things. We know that there are no goblins or monsters under the bed or in the wardrobe. In one sense, the child may know this too. But this doesn't stop him or her being unable to go to sleep at night. Fear manifests itself in this example in ways that are very real that means we should not dismiss fear as an emotion lightly.