Basically, an extrovert, also spelled extravert, is a person who is outgoing, assertive, and very social. Extroversion and its opposite, introversion, form the ends of a continuum that describes one aspect of every person's personality. The concept of extroversion and introversion was popularized by the work of psychologist Carl Jung.
Jung described an extrovert as a person whose psychic energy is directed outward. While most modern day psychologists do not believe in the existence of "psychic energy" per se, they agree that an extrovert is more focused on things external to the self - other people, exciting situations, and the like. An extrovert may feel bored when alone and can typically accomplish more when working in a group. A common stereotype of the extrovert is "the life of the party."
In Western culture, particularly in the United States, the extrovert is considered socially superior to the introvert, as he or she genuinely enjoys the company of others in large groups. The extrovert is often perceived as engaging and exciting. A study conducted by psychologist David Myers found that extroverts are more likely to be "happy" than introverts, although the reason for this correlation is unknown. Other studies have revealed that the extrovert experiences more blood flow in parts of the brain dealing with the senses and emotional response.
There are some drawbacks to being an extrovert, however. Extroverts may have a harder time at school, particularly higher education, because of the amount of solitary studying often required to do well. An extrovert will probably enjoy socializing with peers more than reading books and taking notes alone in his or her room. In addition, extroverts have a higher incidence of juvenile delinquency, probably because of their drive to seek out excitement. Extroverts also have a more powerful response to dopamine than introverts, making some more prone to drug abuse.
If you are unsure whether or not you are an extrovert, there are self-tests available in many books and online to help you determine where you fall along the spectrum. Keep in mind, however, that most people are somewhat of a combination of introvert and extrovert.