" Fear "
A long time ago when our ancestors lived, man used to use his instincts to collect food and for survival, and if man didn't have the function in his mind called fear so that he can run from predators you may have not been here now.
When man at that time faced some kind of danger, his body turns into a state called fight or flight response. Adrenaline is pumped into his blood in order to give him the ability either to start a fight or run away as fast as he can.
This fight and flight response causes many changes like withdrawing blood from sensitive parts like stomach so that you can bear much higher pain, heart beats faster, blood is pumped into muscles so they get stronger and allow you to do more than you can normally do. Your whole system is being in an alert state to face any danger, so looking at the situation from another perspective, when you are afraid, you are much stronger than when you are not!! The only problem with fight or flight response is when it is incorrectly triggered, like being launched just before you stand up for making a speech or a presentation.
Here is where the disorder comes into play, the incorrect perception of the situation caused you to think that you are in danger so you became very anxious and afraid and you may have fear next time of doing the same thing again. So after all, fears are here to protect you from things that you think that you can't handle or from things that can cause you some kind of harm. There are many types and forms of fear, these include fear of rejection, fear of change, phobias with all of its forms (like fear of spiders, heights, dogs, animals, darkness or closed places), fear of failure and even fear of success.
Some fears are culture created, that is you learned it, you were thought to be afraid of certain things that actually may posses no big danger and if you were left without being told that such things are dangerous you may have not feared them as a grown up.
One of the methods you can use to get rid of your fears is exposure therapy, where you expose yourself to the feared situation over and over until you find that there is nothing dangerous about. But remember this can only be used when the fear is arising from an irrational idea, like being afraid of darkness or ghosts.