As with most intangible concepts, emotions have proven hard to pin down in terms of an official definition. In general, emotions are feeling states that accompany certain physiological responses. Evolutionary psychologists see emotion as a survival response that's needed whenever two or more other faculties (like sleep, arousal, or reasoning) are called upon at the same time. These feeling states play an essential role in shaping personality characteristics, as well as determining the likelihood of mental disorder.
When feelings of anger, or happiness, or love surface, the part of the brain called the limbic system is generating these emotion. The limbic system is situated in the mid-brain region, between the brainstem structures and the cortex region. This area is responsible for interpreting an emotion, and directing the behaviors that follow. Information from this area is forwarded to the cortex for processing along with the emotional charge that was generated. In effect, the emotional tone of the information sent will determine how readily the cortex receives, or learns this new information.
The information sent to the limbic system comes from the five senses -vision, hearing, touch, taste and smell. How the limbic system encodes this information will depend on past experiences had with similar information. Not only does it send "feeling" information to the cortex, but it tells the cortex how motivated it needs to be to learn this incoming data. In other words, any information received by the cortex that has a positive emotion attached to it will be readily learned, while information that carries a negative tag will rather impede learning ability.
Early in the 1990's, emotions and their impact on overall intelligence became a topic of inquiry and study. Also referred to as social intelligence, the ability to use one's own emotions, as well as those of others, to her advantage was seen as a different form of intelligence that had more of an impact within a person's life than the traditional measure of intelligence. Self awareness, self motivation, empathy, managing one's own emotions, and managing relationships with others are the areas on which emotional intelligence is focused. A person's "EI" quotient is said to indicate what level of success is possible.
The relationship between emotions and how the brain interprets sensory input is a powerful one. Past learning experiences play a big role in how a person thinks, and determine what emotions that come to the surface in the face of new experiences. A number of psychotherapy approaches are based on treating the thought processes which underlie emotional disorders. And while there may be biological factors at play in the case of severe mood disorders, medication treatment is almost always accompanied by psychotherapy interventions.