Fear is an unpleasant feeling of perceived risk, real or not, or danger. It's one of the basic emotions (see emotion theory).
Fear is a very powerful tool for behavior modification.
Quote: "We have nothing to fear but fear itself" (Franklin D. Roosevelt)
The opposite is confidence. Self-confidence is confidence in oneself, i.e. in one's abilities and worth, including the abilitiy to cope with one's limitations and problems.
Unjustified confidence can cause problems.