Evil is an ambiguous word generally describing that which is morally bad, wrong, wicked (by some moral standard); it also describes conditions that are painful, ruinous, or disastrous. It is one half of the duality of good and evil expressed, in some form or another, by all cultures.
The concept of natural evil refers to things not caused by acts of human will, such as natural disasters, death, pain, and disease. Moral evil refers to human acts that are judged to be morally wicked.
Definitions of good and evil, and what specific things are good or evil, fall into two major classes: moral absolutism, which holds that good and evil are fixed concepts of reality, established by God, nature, or some other authority. Moral relativism holds that standards of good and evil are only products of local culture, custom, or prejudice.
Regardless of the source of their definitions, most cultures largely agree upon many things as evil: natural evils generally include accidental death, disease, and other misfortunes. Moral evils generally include violence toward others (though perhaps not to "outsiders" of the group), and deceitfulness. "Taking advantage of another person for one's own benefit" is the Unification Church definition of evil.
The Abrahamic religions, as well as others, are largely centered around the concepts of good and evil, and this has lead to much religious debate. Many cultures and mythologies personify evil, such as with Satan in Christianity. Others describe evil spirits or demons as the inciters of acts.
Many cultures recognize many levels of immoral behavior, from minor vices to major crimes. These beliefs are often encoded into the laws of a society, with methods of judgment and punishment for offenses.
See also: goodness, religion, philosophy, law, The problem of evil.
Why Evil Exists
Evil is needed in the world, sometimes more than good is. Evil counteracts good, forcing the world to remain neutral and stopping it from ending. What most people consider evil is usually done by people who would be considered good at any other time. People constantly confuse evil with unlawfulness, which is very troubling for me, since I am evil.