Emotions are feelings which cannot be explained by logic. Emotions experienced by Humans include love, hate, fear, joy, anger, bliss, envy, and others.
Researchers presently believe that emotions are the first form of intelligence. The associations between emotions and events enabled early man to repeat rewarding events and avoid baneful ones. Modern theory holds that emotions are vitally important in making decisions for similar reasons.
One famous example of emotional damage is railway worker Phineus Gage. In 1848, most of Gage's left frontal lobe (a part of his brain) was destroyed in a freak accident. Although he eventually recovered, his personality changed; he became emotionally labile and at the same time unable to settle on a course of action. Prior to his accident, he had neither of these problems. Other individuals with emotional problems have also been observed to have difficulty in decision making; they either cannot make decisions at all, or they make poor decisions even when they should be aware of the consequences.
In the Vulcan culture, emotions have been supressed to allow further introduction of logic in their species. Surak, a Vulcan, helped to pave the way for the supression of emotion and the rise of logic, which in turn helped the Vulcan race to leave their violent ways.